Oracle Architecture------------------------------------------------------------------------------------------------------------------------------------------

Oracle server: An Oracle server includes an Oracle Instance and an Oracle database. Oracle database: An Oracle database consists of files. Sometimes these are referred to as operating system files, but they are actually database files that store the database information that a firm or organization needs in order to operate. The redo log files are used to recover the database in the event of application program failures, instance failures and other minor failures. The archived redo log files are used to recover the database if a disk fails. Other files not shown in the figure include:o The required parameter file that is used to specify parameters for configuring an Oracle instance when it starts up. o The optional password file authenticates special users of the database – these are termed privileged users and include database administrators. o Alert and Trace Log Files – these files store information about errors and actions taken that affect the configuration of the database

Oracle instance: An Oracle Instance consists of two different sets of components: The first component set is the set of background processes (PMON, SMON, RECO, DBW0, LGWR, CKPT, D000 and others). o These will be covered later in detail – each background process is a computer program. o These processes perform input/output and monitor other Oracle processes to provide good performance and database reliability. The second component set includes the memory structures that comprise the Oracle instance. o When an instance starts up, a memory structure called the System Global Area (SGA) is allocated. o At this point the background processes also start. An Oracle Instance provides access to one and only one Oracle database.

Storage Structure

An Oracle database is made up of physical and logical structures. Physical structures can be seen and operated on from the operating system, such as the physical files that store data on a disk.Logical structures are created and recognized by Oracle Database and are not known to the operating system. The primary logical structure in a database, a tablespace, contains physical files. The applications developer or user may be aware of the logical structure, but is not usually aware of this physical structure. The database administrator (DBA) must understand the relationship between the physical and logical structures of a database.

Logical Storage Structures

This section discusses logical storage structures. The following logical storage structures enable Oracle Database to have fine-grained control of disk space use:TablespaceA database is divided into logical storage units called tablespaces. A tablespace is the logical container for a segment. Each tablespace contains at least one data file.SegmentA segment is a set of extents allocated for a user object (for example, a table or index), undo data , or temporary data.

ExtentAn extent is a specific number of logically contiguous data blocks, obtained in a single allocation, used to store a specific type of information.Data blocksAt the finest level of granularity, Oracle Database data is stored in data blocks. One data block corresponds to a specific number of bytes on disk.

Table spaces___________________________A database consists of one or more tablespaces. A tablespace is a logical grouping of one or more physical datafiles or tempfiles, and is the primary structure by which the database manages storage.There are various types of tablespaces, including the following: Permanent tablespacesThese tablespaces are used to store system and user data. Permanent tablespaces consist of one or more datafiles. In Oracle Database XE, all your application data is by default stored in the tablespace named USERS. This tablespace consists of a single datafile that automatically grows (autoextends) as your applications store more data.Online and Offline TablespacesA tablespace can be online (accessible) or offline (not accessible). A tablespace is generally online, so that users can access the information in the tablespace. However, sometimes a tablespace is taken offline to make a portion of the database unavailable while allowing normal access to the remainder of the database. This makes many administrative tasks easier to perform.

Temporary tablespacesTemporary tablespaces improve the concurrency of multiple sort operations, and reduce their overhead. Temporary tablespaces are the most efficient tablespaces for disk sorts. Temporary tablespaces consist of one or more tempfiles. Oracle Database XE automatically manages storage for temporary tablespaces. Undo tablespace

Oracle Database XE transparently creates and automatically manages undo data in this tablespace.When a transaction modifies the database, Oracle Database XE makes a copy of the original data before modifying it. The original copy of the modified data is called undo data. This information is necessary for the following reasons:

To undo any uncommitted changes made to the database in the event that a rollback operation is necessary. A rollback operation can be the result of a user specifically issuing a ROLLBACK statement to undo the changes of a misguided or unintentional transaction, or it can be part of a recovery operation.To provide read consistency, which means that each user can get a consistent view of data, even while other uncommitted changes may be occurring against the data. For example, if a user issues a query at 10:00 a.m. and the query runs for 15 minutes, then the query results should reflect the entire state of the data at 10:00 a.m., regardless of updates or inserts by other users during the query.The SYSTEM TablespaceThe SYSTEM tablespace is a necessary administrative tablespace included with the database when it is created. Oracle Database uses SYSTEM to manage the database.The SYSTEM tablespace includes the following information, all owned by the SYS user:The data dictionaryTables and views that contain administrative information about the databaseCompiled stored objects such as triggers, procedures, and packagesThe SYSTEM tablespace is managed as any other tablespace, but requires a higher level of privilege and is restricted in some ways. For example, you cannot rename or drop the SYSTEM tablespace.By default, Oracle Database sets all newly created user tablespaces to be locally managed. In a database with a locally managed SYSTEM tablespace, you cannot create dictionary-managed tablespaces (which are deprecated). However, if you execute the CREATE DATABASE statement manually and accept the defaults, then the SYSTEM tablespace is dictionary managed. You can migrate an existing dictionary-managed SYSTEM tablespace to a locally managed format.

The SYSAUX TablespaceThe SYSAUX tablespace is an auxiliary tablespace to the SYSTEM tablespace. The SYSAUX tablespace provides a centralized location for database metadata that does not reside in the SYSTEM tablespace. It reduces the number of tablespaces created by default, both in the seed database and in user-defined databases.Several database components, including Oracle Enterprise Manager and Oracle Streams, use the SYSAUX tablespace as their default storage location. Therefore, the SYSAUX tablespace is created automatically during database creation or upgrade.

During normal database operation, the database does not allow the SYSAUX tablespace to be dropped or renamed. If the SYSAUX tablespace becomes unavailable, then core database functionality remains operational. The database features that use the SYSAUX tablespace could fail, or function with limited capability.

Undo TablespacesAn undo tablespace is a locally managed tablespace reserved for system-managed undo data (see "Undo Segments"). Like other permanent tablespaces, undo tablespaces contain data files. Undo blocks in these files are grouped in extents.Automatic Undo Management ModeUndo tablespaces require the database to be in the default automatic undo management mode. This mode eliminates the complexities of manually administering undo segments. The database automatically tunes itself to provide the best possible retention of undo data to satisfy long-running queries that may require this data.An undo tablespace is automatically created with a new installation of Oracle Database. Earlier versions of Oracle Database may not include an undo tablespace and use legacy rollback segments instead, known as manual undo management mode. When upgrading to Oracle Database 11g, you can enable automatic undo management mode and create an undo tablespace. Oracle Database contains an Undo Advisor that provides advice on and helps automate your undo environment.A database can contain multiple undo tablespaces, but only one can be in use at a time. When an instance attempts to open a database, Oracle Database automatically selects the first available undo tablespace. If no undo tablespace is available, then the instance starts without an undo tablespace and stores undo data in the SYSTEM tablespace. Storing undo data in SYSTEM is not recommended.Automatic Undo RetentionThe undo retention period is the minimum amount of time that Oracle Database attempts to retain old undo data before overwriting it. Undo retention is important because long-running queries may require older block images to supply read consistency. Also, some Oracle Flashback features can depend on undo availability.In general, it is desirable to retain old undo data as long as possible. After a transaction commits, undo data is no longer needed for rollback or transaction

recovery. The database can retain old undo data if the undo tablespace has space for new transactions. When available space is low, the database begins to overwrite old undo data for committed transactions.Oracle Database automatically provides the best possible undo retention for the current undo tablespace. The database collects usage statistics and tunes the retention period based on these statistics and the undo tablespace size. If the undo tablespace is configured with the AUTOEXTEND option, and if the maximum size is not specified, then undo retention tuning is different. In this case, the database tunes the undo retention period to be slightly longer than the longest-running query, if space allows.Read ConsistencyRead consistency, as supported by Oracle, does the following:Guarantees that the set of data seen by a statement is consistent with respect to a single point in time and does not change during statement execution (statement-level read consistency)Ensures that readers of database data do not wait for writers or other readers of the same dataEnsures that writers of database data do not wait for readers of the same dataEnsures that writers only wait for other writers if they attempt to update identical rows in concurrent transactionsThe simplest way to think of Oracle's implementation of read consistency is to imagine each user operating a private copy of the database, hence the multiversion consistency model.Read Consistency, Undo Records, and TransactionsTo manage the multiversion consistency model, Oracle must create a read-consistent set of data when a table is queried (read) and simultaneously updated (written). When an update occurs, the original data values changed by the update are recorded in the database undo records. As long as this update remains part of an uncommitted transaction, any user that later queries the modified data views the original data values. Oracle uses current information in the system global area and information in the undo records to construct a read-consistent view of a table's data for a query.Only when a transaction is committed are the changes of the transaction made permanent. Statements that start after the user's transaction is committed only see the changes made by the committed transaction.The transaction is key to Oracle's strategy for providing read consistency. This unit of committed (or uncommitted) SQL statements:

Dictates the start point for read-consistent views generated on behalf of readersControls when modified data can be seen by other transactions of the database for reading or updating

Temporary TablespacesA temporary tablespace contains transient data that persists only for the duration of a session. No permanent schema objects can reside in a temporary tablespace. The database stores temporary tablespace data in temp files.Temporary tablespaces can improve the concurrency of multiple sort operations that do not fit in memory. These tablespaces also improve the efficiency of space management operations during sorts.When the SYSTEM tablespace is locally managed, a default temporary tablespace is included in the database by default during database creation. A locally managed SYSTEM tablespace cannot serve as default temporary storage.You can specify a user-named default temporary tablespace when you create a database by using the DEFAULT TEMPORARY TABLESPACE extension to the CREATE DATABASE statement. If SYSTEM is dictionary managed, and if a default temporary tablespace is not defined at database creation, then SYSTEM is the default temporary storage. However, the database writes a warning in the alert log saying that a default temporary tablespace is recommended.

Managing Space in TablespacesTablespaces allocate space in extents. Tablespaces can use two different methods to keep track of their free and used space:Locally managed tablespaces: Extent management by the tablespaceDictionary managed tablespaces: Extent management by the data dictionaryWhen you create a tablespace, you choose one of these methods of space management. Later, you can change the management method with the DBMS_SPACE_ADMIN PL/SQL package.Locally Managed TablespacesA tablespace that manages its own extents maintains a bitmap in each datafile to keep track of the free or used status of blocks in that datafile. Each bit in the bitmap corresponds to a block or a group of blocks. When an extent is allocated or freed for reuse, Oracle changes the bitmap values to show the new status of the blocks. These changes do not generate rollback information because they do not update tables in the data dictionary (except for special cases such as tablespace quota information).

Locally managed tablespaces have the following advantages over dictionary managed tablespaces:Local management of extents automatically tracks adjacent free space, eliminating the need to coalesce free extents.Local management of extents avoids recursive space management operations. Such recursive operations can occur in dictionary managed tablespaces if consuming or releasing space in an extent results in another operation that consumes or releases space in a data dictionary table or rollback segment.The sizes of extents that are managed locally can be determined automatically by the system. Alternatively, all extents can have the same size in a locally managed tablespace and override object storage options.The LOCAL clause of the CREATE TABLESPACE or CREATE TEMPORARY TABLESPACE statement is specified to create locally managed permanent or temporary tablespaces, respectively.Segment Space Management in Locally Managed TablespacesWhen you create a locally managed tablespace using the CREATE TABLESPACE statement, the SEGMENT SPACE MANAGEMENT clause lets you specify how free and used space within a segment is to be managed. Your choices are:AUTOThis keyword tells Oracle that you want to use bitmaps to manage the free space within segments. A bitmap, in this case, is a map that describes the status of each data block within a segment with respect to the amount of space in the block available for inserting rows. As more or less space becomes available in a data block, its new state is reflected in the bitmap. Bitmaps enable Oracle to manage free space more automatically; thus, this form of space management is called automatic segment-space management.Locally managed tablespaces using automatic segment-space management can be created as smallfile (traditional) or bigfile tablespaces. AUTO is the default.MANUALThis keyword tells Oracle that you want to use free lists for managing free space within segments. Free lists are lists of data blocks that have space available for inserting rows.Dictionary Managed TablespacesIf you created your database with an earlier version of Oracle, then you could be using dictionary managed tablespaces. For a tablespace that uses the data dictionary to manage its extents, Oracle updates the appropriate tables in the data dictionary whenever an extent is allocated or freed for reuse. Oracle also

stores rollback information about each update of the dictionary tables. Because dictionary tables and rollback segments are part of the database, the space that they occupy is subject to the same space management operations as all other data.Multiple Block SizesOracle supports multiple block sizes in a database. The standard block size is used for the SYSTEM tablespace. This is set when the database is created and can be any valid size. You specify the standard block size by setting the initialization parameter DB_BLOCK_SIZE. Legitimate values are from 2K to 32K.In the initialization parameter file or server parameter, you can configure subcaches within the buffer cache for each of these block sizes. Subcaches can also be configured while an instance is running. You can create tablespaces having any of these block sizes. The standard block size is used for the system tablespace and most other tablespaces.

Tablespace ModesThe tablespace mode determines the accessibility of the tablespace.Read/Write and Read-Only TablespacesEvery tablespace is in a write mode that specifies whether it can be written to. The mutually exclusive modes are as follows:Read/write modeUsers can read and write to the tablespace. All tablespaces are initially created as read/write. The SYSTEM and SYSAUX tablespaces and temporary tablespaces are permanently read/write, which means that they cannot be made read-only.Read-only modeWrite operations to the data files in the tablespace are prevented. A read-only tablespace can reside on read-only media such as DVDs or WORM drives.Read-only tablespaces eliminate the need to perform backup and recovery of large, static portions of a database. Read-only tablespaces do not change and thus do not require repeated backup. If you recover a database after a media failure, then you do not need to recover read-only tablespaces.Online and Offline TablespacesA tablespace can be online (accessible) or offline (not accessible) whenever the database is open. A tablespace is usually online so that its data is available to users. The SYSTEM tablespace and temporary tablespaces cannot be taken offline.

A tablespace can go offline automatically or manually. For example, you can take a tablespace offline for maintenance or backup and recovery. The database automatically takes a tablespace offline when certain errors are encountered, as when the database writer (DBW) process fails in several attempts to write to a data file. Users trying to access tables in an offline tablespace receive an error.When a tablespace goes offline, the database does the following:The database does not permit subsequent DML statements to reference objects in the offline tablespace. An offline tablespace cannot be read or edited by any utility other than Oracle Database.Active transactions with completed statements that refer to data in that tablespace are not affected at the transaction level.The database saves undo data corresponding to those completed statements in a deferred undo segment in the SYSTEM tablespace. When the tablespace is brought online, the database applies the undo data to the tablespace, if needed.

Tablespace File SizeA tablespace is either a bigfile tablespace or a smallfile tablespace. These tablespaces are indistinguishable in terms of execution of SQL statements that do not explicitly refer to data files or temp files. The difference is as follows:A smallfile tablespace can contain multiple data files or temp files, but the files cannot be as large as in a bigfile tablespace. This is the default tablespace type.A bigfile tablespace contains one very large data file or temp file. This type of tablespaces can do the following:Increase the storage capacity of a databaseThe maximum number of data files in a database is limited (usually to 64 KB files), so increasing the size of each data file increases the overall storage.Reduce the burden of managing many data files and temp filesBigfile tablespaces simplify file management with Oracle Managed Files and Automatic Storage Management (Oracle ASM) by eliminating the need for adding new files and dealing with multiple files.Perform operations on tablespaces rather than individual filesBigfile tablespaces make the tablespace the main unit of the disk space administration, backup and recovery, and so on.Bigfile tablespaces are supported only for locally managed tablespaces with ASSM. However, locally managed undo and temporary tablespaces can be bigfile tablespaces even when segments are manually managed.

SEGMENTA segment is a set of extents that contains all the data for a specific logical storage structure within a tablespace. For example, for each table, Oracle allocates one or more extents to form that table's data segment, and for each index, Oracle allocates one or more extents to form its index segment.Oracle Database manages segment space automatically or manually. This section assumes the use of ASSM.Types of segmentUser SegmentsA single data segment in a database stores the data for one user object. There are different types of segments. Examples of user segments include:Table, table partition, or table cluster

Multiple segment

Temporary Segments:When processing a query, Oracle Database often requires temporary workspace for intermediate stages of SQL statement execution. Typical operations that may

require a temporary segment include sorting, hashing, and merging bitmaps. While creating an index, Oracle Database also places index segments into temporary segments and then converts them into permanent segments when the index is complete.Oracle Database does not create a temporary segment if an operation can be performed in memory. However, if memory use is not possible, then the database automatically allocates a temporary segment on disk

3. Undo SegmentsOracle Database maintains records of the actions of transactions, collectively known as undo data. Oracle Database uses undo to do the following:Roll back an active transactionRecover a terminated transactionProvide read consistencyPerform some logical flashback operationsOracle Database stores undo data inside the database rather than in external logs. Undo data is stored in blocks that are updated just like data blocks, with changes to these blocks generating redo. In this way, Oracle Database can efficiently access undo data without needing to read external logs.Undo data is stored in an undo tablespace. Oracle Database provides a fully automated mechanism, known as automatic undo management mode, for managing undo segments and space in an undo tablespace.Undo Segments and TransactionsWhen a transaction starts, the database binds (assigns) the transaction to an undo segment, and therefore to a transaction table, in the current undo tablespace. In rare circumstances, if the database instance does not have a designated undo tablespace, then the transaction binds to the system undo segment.

ExtentsAn extent is a logical unit of database storage space allocation made up of a number of contiguous data blocks. One or more extents in turn make up a segment. When the existing space in a segment is completely used, Oracle allocates a new extent for the segment.When Extents Are AllocatedWhen you create a table, Oracle allocates to the table's data segment an initial extent of a specified number of data blocks. Although no rows have been inserted yet, the Oracle data blocks that correspond to the initial extent are reserved for that table's rows.

If the data blocks of a segment's initial extent become full and more space is required to hold new data, Oracle automatically allocates an incremental extent for that segment. An incremental extent is a subsequent extent of the same or greater size than the previously allocated extent in that segment.For maintenance purposes, the header block of each segment contains a directory of the extents in that segment.

Db Block: The Oracle Server manages data at the smallest unit in what is termed a block or data block. Data are actually stored in blocks.

Block headerThis part contains general information about the block, including disk address and segment type. For blocks that are transaction-managed, the block header contains active and historical transaction information.A transaction entry is required for every transaction that updates the block. Oracle Database initially reserves space in the block header for transaction entries. In data blocks allocated to segments that support transactional changes, free space can also hold transaction entries when the header space is depleted. The space required for transaction entries is operating system dependent.

However, transaction entries in most operating systems require approximately 23 bytes.Table directoryFor a heap-organized table, this directory contains metadata about tables whose rows are stored in this block. Multiple tables can store rows in the same block.Row directoryFor a heap-organized table, this directory describes the location of rows in the data portion of the block.After space has been allocated in the row directory, the database does not reclaim this space after row deletion. Thus, a block that is currently empty but formerly had up to 50 rows continues to have 100 bytes allocated for the row directory. The database reuses this space only when new rows are inserted in the block.Row DataThis portion of the data block contains table or index data. Rows can span blocks.Free SpaceFree space is allocated for insertion of new rows and for updates to rows that require additional space (for example, when a trailing null is updated to a nonnull value).In data blocks allocated for the data segment of a table or cluster, or for the index segment of an index, free space can also hold transaction entries. A transaction entry is required in a block for each INSERT, UPDATE, DELETE, and SELECT...FOR UPDATE statement accessing one or more rows in the block. The space required for transaction entries is operating system dependent; however, transaction entries in most operating systems require approximately 23 bytes.

Datafile: Tablespaces are stored in datafiles which are physical disk objects. A datafile can only store objects for a single tablespace, but a tablespace may have more than one datafile – this happens when a disk drive device fills up and a tablespace needs to be expanded, then it is expanded to a new disk drive. The DBA can change the size of a datafile to make it smaller or later. The file can also grow in size dynamically as the tablespace grows.Thus, the Oracle database architecture includes both logical and physical structures as follows: Physical: Control files; Redo Log Files; Datafiles; Operating System Blocks. Logical: Tablespaces; Segments; Extents; Data Blocks.

Physical Structure

 

As was noted above, an Oracle database consists of physical files. The database itself has:

        Datafiles – these contain the organization's actual data.

A tablespace in an Oracle database consists of one or more physical datafiles. A datafile can be associated with only one tablespace and only one database.

Oracle creates a datafile for a tablespace by allocating the specified amount of disk space plus the overhead required for the file header. When a datafile is created, the operating system under which Oracle runs is responsible for clearing old information and authorizations from a file before allocating it to Oracle. If the file is large, this process can take a significant amount of time. The first tablespace in any database is always the SYSTEM tablespace, so Oracle automatically allocates the first datafiles of any database for the SYSTEM tablespace during database creation.

Datafile Contents

When a datafile is first created, the allocated disk space is formatted but does not contain any user data. However, Oracle reserves the space to hold the data for future segments of the associated tablespace—it is used exclusively by Oracle. As the data grows in a tablespace, Oracle uses the free space in the associated datafiles to allocate extents for the segment.

The data associated with schema objects in a tablespace is physically stored in one or more of the datafiles that constitute the tablespace. Note that a schema object does not correspond to a specific datafile; rather, a datafile is a repository for the data of any schema object within a specific tablespace. Oracle allocates space for the data associated with a schema object in one or more datafiles of a tablespace. Therefore, a schema object can span one or more datafiles. Unless table striping is used (where data is spread across more than one disk), the database administrator and end users cannot control which datafile stores a schema object.

Size of Datafiles

You can alter the size of a datafile after its creation or you can specify that a datafile should dynamically grow as schema objects in the tablespace grow. This functionality enables you to have fewer datafiles for each tablespace and can simplify administration of datafiles.

Temporary Datafiles

Locally managed temporary tablespaces have temporary datafiles (tempfiles), which are similar to ordinary datafiles, with the following exceptions:

Tempfiles are always set to NOLOGGING mode. You cannot make a tempfile read only. You cannot create a tempfile with the ALTER DATABASE statement. Media recovery does not recognize tempfiles:

o BACKUP CONTROLFILE does not generate any information for tempfiles.

o CREATE CONTROLFILE cannot specify any information about tempfiles.

When you create or resize tempfiles, they are not always guaranteed allocation of disk space for the file size specified. On certain file systems (for example, UNIX) disk blocks are allocated not at file creation or resizing, but before the blocks are accessed.

Control Files

The database control file is a small binary file necessary for the database to start and operate successfully. A control file is updated continuously by Oracle during database use, so it must be available for writing whenever the database is open. If for some reason the control file is not accessible, then the database cannot function properly.

Each control file is associated with only one Oracle database.

Control File Contents

A control file contains information about the associated database that is required for access by an instance, both at startup and during normal operation. Control file

information can be modified only by Oracle; no database administrator or user can edit a control file.

Among other things, a control file contains information such as:

The database name The timestamp of database creation The names and locations of associated datafiles and redo log files Tablespace information Datafile offline ranges The log history Archived log information Backup set and backup piece information Backup datafile and redo log information Datafile copy information The current log sequence number Checkpoint information

The database name and timestamp originate at database creation. The database name is taken from either the name specified by the DB_NAME initialization parameter or the name used in the CREATE DATABASE statement.

Each time that a datafile or a redo log file is added to, renamed in, or dropped from the database, the control file is updated to reflect this physical structure change. These changes are recorded so that:

Oracle can identify the datafiles and redo log files to open during database startup

Oracle can identify files that are required or available in case database recovery is necessary

Therefore, if you make a change to the physical structure of your database (using ALTER DATABASE statements), then you should immediately make a backup of your control file.

Control files also record information about checkpoints. Every three seconds, the checkpoint process (CKPT) records information in the control file about the checkpoint position in the redo log. This information is used during database recovery to tell Oracle that all redo entries recorded before this point in the redo

log group are not necessary for database recovery; they were already written to the datafiles.

Multiplexed Control Files

As with redo log files, Oracle enables multiple, identical control files to be open concurrently and written for the same database. By storing multiple control files for a single database on different disks, you can safeguard against a single point of failure with respect to control files. If a single disk that contained a control file crashes, then the current instance fails when Oracle attempts to access the damaged control file. However, when other copies of the current control file are available on different disks, an instance can be restarted without the need for database recovery.

If all control files of a database are permanently lost during operation, then the instance is aborted and media recovery is required. Media recovery is not straightforward if an older backup of a control file must be used because a current copy is not available. It is strongly recommended that you adhere to the following:

Use multiplexed control files with each database Store each copy on a different physical disk Use operating system mirroring Monitor backups

The control file serves the following purposes:

It contains information about data files, online redo log files, and so on that are required to open the database.

The control file tracks structural changes to the database. For example, when an administrator adds, renames, or drops a data file or online redo log file, the database updates the control file to reflect this change.

It contains metadata that must be accessible when the database is not open.

For example, the control file contains information required to recover the database, including checkpoints. A checkpoint indicates the SCN in the redo stream where instance recovery would be required to begin Every committed change before a checkpoint SCN is guaranteed to be saved on disk in the data files. At least every three seconds the checkpoint process records

information in the control file about the checkpoint position in the online redo log.

Oracle Database reads and writes to the control file continuously during database use and must be available for writing whenever the database is open. For example, recovering a database involves reading from the control file the names of all the data files contained in the database. Other operations, such as adding a data file, update the information stored in the control file

Online Redo Log

The most crucial structure for recovery is the online redo log, which consists of two or more preallocated files that store changes to the database as they occur. The online redo log records changes to the data files.

Use of the Online Redo LogThe database maintains online redo log files to protect against data loss. Specifically, after an instance failure the online redo log files enable Oracle Database to recover committed data not yet written to the data files.

Oracle Database writes every transaction synchronously to the redo log buffer, which is then written to the online redo logs. The contents of the log include uncommitted transactions, undo data, and schema and object management statements.

Oracle Database uses the online redo log only for recovery. However, administrators can query online redo log files through a SQL interface in the Oracle LogMiner utility Redo log files are a useful source of historical information about database activity.

How Oracle Database Writes to the Online Redo Log

The online redo log for a database instance is called a redo thread. In single-instance configurations, only one instance accesses a database, so only one redo thread is present. In an Oracle Real Application Clusters (Oracle RAC) configuration, however, two or more instances concurrently access a database, with

each instance having its own redo thread. A separate redo thread for each instance avoids contention for a single set of online redo log files.

An online redo log consists of two or more online redo log files. Oracle Database requires a minimum of two files to guarantee that one is always available for writing while the other is being archived (if the database is in ARCHIVELOG mode).

Online Redo Log Switches

Oracle Database uses only one online redo log file at a time to store records written from the redo log buffer. The online redo log file to which the log writer (LGWR) process is actively writing is called the current online redo log file.

A log switch occurs when the database stops writing to one online redo log file and begins writing to another. Normally, a switch occurs when the current online redo log file is full and writing must continue. However, you can configure log switches to occur at regular intervals, regardless of whether the current online redo log file is filled, and force log switches manually.

Log writer writes to online redo log files circularly. When log writer fills the last available online redo log file, the process writes to the first log file, restarting the cycle.

Multiple Copies of Online Redo Log Files

Oracle Database can automatically maintain two or more identical copies of the online redo log in separate locations. An online redo log group consists of an online redo log file and its redundant copies. Each identical copy is a member of the online redo log group. Each group is defined by a number, such as group 1, group 2, and so on.

Maintaining multiple members of an online redo log group protects against the loss of the redo log. Ideally, the locations of the members should be on separate disks so that the failure of one disk does not cause the loss of the entire online redo log.

In , A_LOG1 and B_LOG1 are identical members of group 1, while A_LOG2 and B_LOG2 are identical members of group 2. Each member in a group must be the same size. LGWR writes concurrently to group 1 (members A_LOG1 and B_LOG1), then writes concurrently to group 2 (members A_LOG2 and B_LOG2),

then writes to group 1, and so on. LGWR never writes concurrently to members of different groups.

Oracle recommends that you multiplex the online redo log. The loss of log files can be catastrophic if recovery is required. When you multiplex the online redo log, the database must increase the amount of I/O it performs. Depending on your system, this additional I/O may impact overall database performance.

Structure of the Online Redo Log

Online redo log files contain redo records. A redo record is made up of a group of change vectors, each of which describes a change to a data block. For example, an update to a salary in the employees table generates a redo record that describes changes to the data segment block for the table, the undo segment data block, and the transaction table of the undo segments.

The redo records have all relevant metadata for the change, including the following:

SCN and time stamp of the change Transaction ID of the transaction that generated the change SCN and time stamp when the transaction committed (if it committed) Type of operation that made the change

Name and type of the modified data segment

Archived Redo Log Files

An archived redo log file is a copy of a filled member of an online redo log group. This file is not considered part of the database, but is an offline copy of an online redo log file created by the database and written to a user-specified location.

Archived redo log files are a crucial part of a backup and recovery strategy. You can use archived redo log files to:

Recover a database backup Update a standby database (see "Computer Failures")

Archiving is the operation of generating an archived redo log file. Archiving is either automatic or manual and is only possible when the database is in ARCHIVELOG mode.

An archived redo log file includes the redo entries and the log sequence number of the identical member of the online redo log group. In files A_LOG1 and B_LOG1 are identical members of Group 1. If the database is in ARCHIVELOG mode, and if automatic archiving is enabled, then the archiver process (ARCn) will archive one of these files. If A_LOG1 is corrupted, then the process can archive B_LOG1. The archived redo log contains a copy of every group created since you enabled archiving.

Parameter Files

o start a database instance, Oracle Database must read either a server parameter file, which is recommended, or a text initialization parameter file, which is a legacy implementation. These files contain a list of configuration parameters.

To create a database manually, you must start an instance with a parameter file and then issue a CREATE DATABASE command. Thus, the instance and parameter file can exist even when the database itself does not exist.

Initialization Parameters

Initialization parameters are configuration parameters that affect the basic operation of an instance. The instance reads initialization parameters from a file at startup.

Oracle Database provides many initialization parameters to optimize its operation in diverse environments. Only a few of these parameters must be explicitly set because the default values are adequate in most cases.

Functional Groups of Initialization Parameters

Most initialization parameters belong to one of the following functional groups:

Parameters that name entities such as files or directories Parameters that set limits for a process, database resource, or the database

itself Parameters that affect capacity, such as the size of the SGA (these

parameters are called variable parameters)

Variable parameters are of particular interest to database administrators because they can use these parameters to improve database performance.

Basic and Advanced Initialization Parameters

Initialization parameters are divided into two groups: basic and advanced. In most cases, you must set and tune only the approximately 30 basic parameters to obtain reasonable performance. The basic parameters set characteristics such as the database name, locations of the control files, database block size, and undo tablespace.

In rare situations, modification to the advanced parameters may be required for optimal performance. The advanced parameters enable expert DBAs to adapt the behavior of the Oracle Database to meet unique requirements.

Oracle Database provides values in the starter initialization parameter file provided with your database software, or as created for you by the Database Configuration Assistant. You can edit these Oracle-supplied initialization parameters and add others, depending on your configuration and how you plan to tune the database. For relevant initialization parameters not included in the parameter file, Oracle Database supplies defaults.

Server Parameter Files

A server parameter file is a repository for initialization parameters that is managed by Oracle Database. A server parameter file has the following key characteristics:

Only one server parameter file exists for a database. This file must reside on the database host.

The server parameter file is written to and read by only by Oracle Database, not by client applications.

The server parameter file is binary and cannot be modified by a text editor. Initialization parameters stored in the server parameter file are persistent.

Any changes made to the parameters while a database instance is running can persist across instance shutdown and startup.

A server parameter file eliminates the need to maintain multiple text initialization parameter files for client applications. A server parameter file is initially built from a text initialization parameter file using the CREATE SPFILE statement. It can also be created directly by the Database Configuration Assistant.

Text Initialization Parameter Files

A text initialization parameter file is a text file that contains a list of initialization parameters. This type of parameter file, which is a legacy implementation of the parameter file, has the following key characteristics:

When starting up or shutting down a database, the text initialization parameter file must reside on the same host as the client application that connects to the database.

A text initialization parameter file is text-based, not binary. Oracle Database can read but not write to the text initialization parameter

file. To change the parameter values you must manually alter the file with a text editor.

Changes to initialization parameter values by ALTER SYSTEM are only in effect for the current instance. You must manually update the text initialization parameter file and restart the instance for the changes to be known.

db_name=sample control_files=/disk1/oradata/sample_cf.dbf

db_block_size=8192 open_cursors=52 undo_management=auto shared_pool_size=280M pga_aggregate_target=29M

Modification of Initialization Parameter Values

You can adjust initialization parameters to modify the behavior of a database. The classification of parameters as static or dynamicStatic parameters include DB_BLOCK_SIZE, DB_NAME, and COMPATIBLE. Dynamic parameters are grouped into session-level parameters, which affect only the current user session, and system-level parameters, which affect the database and all sessions. For example, MEMORY_TARGET is a system-level parameter, while NLS_DATE_FORMAT is a session-level parameter

The scope of a parameter change depends on when the change takes effect. When an instance has been started with a server parameter file, you can use the ALTER SYSTEM SET statement to change values for system-level parameters as follows:

SCOPE=MEMORY

Changes apply to the database instance only. The change will not persist if the database is shut down and restarted.

SCOPE=SPFILE

Changes are written to the server parameter file but do not affect the current instance. Thus, the changes do not take effect until the instance is restarted.

SCOPE=BOTH

Changes are written both to memory and to the server parameter file. This is the default scope when the database is using a server parameter file.

The database prints the new value and the old value of an initialization parameter to the alert log. As a preventative measure, the database validates changes of basic parameter to prevent illegal values from being written to the server parameter file.

Diagnostic Files

Oracle Database includes a fault diagnosability infrastructure for preventing, detecting, diagnosing, and resolving database problems. Problems include critical errors such as code bugs, metadata corruption, and customer data corruption.

The goals of the advanced fault diagnosability infrastructure are the following:

Detecting problems proactively Limiting damage and interruptions after a problem is detected Reducing problem diagnostic and resolution time Simplifying customer interaction with Oracle Support

Automatic Diagnostic Repository

Automatic Diagnostic Repository (ADR) is a file-based repository that stores database diagnostic data such as trace files, the alert log, and Health Monitor reports. Key characteristics of ADR include:

Unified directory structure Consistent diagnostic data formats Unified tool set

The preceding characteristics enable customers and Oracle Support to correlate and analyze diagnostic data across multiple Oracle instances, components, and products.

ADR is located outside the database, which enables Oracle Database to access and manage ADR when the physical database is unavailable. An instance can create ADR before a database has been created.

Problems and Incidents

ADR proactively tracks problems, which are critical errors in the database. Critical errors manifest as internal errors, such as ORA-600, or other severe errors. Each problem has a problem key, which is a text string that describes the problem.

When a problem occurs multiple times, ADR creates a time-stamped incident for each occurrence. An incident is uniquely identified by a numeric incident ID. When an incident occurs, ADR sends an incident alert to Enterprise Manager. Diagnosis and resolution of a critical error usually starts with an incident alert.

Because a problem could generate many incidents in a short time, ADR applies flood control to incident generation after certain thresholds are reached. A flood-controlled incident generates an alert log entry, but does not generate incident dumps. In this way, ADR informs you that a critical error is ongoing without overloading the system with diagnostic data.

See Also:

Oracle Database Administrator's Guide for detailed information about the fault diagnosability infrastructure

ADR Structure

The ADR base is the ADR root directory. The ADR base can contain multiple ADR homes, where each ADR home is the root directory for all diagnostic data—traces, dumps, the alert log, and so on—for an instance of an Oracle product or component. For example, in an Oracle RAC environment with shared storage and ASM, each database instance and each ASM instance has its own ADR home.

illustrates the ADR directory hierarchy for a database instance. Other ADR homes for other Oracle products or components, such as ASM or Oracle Net Services, can exist within this hierarchy, under the same ADR base.

ADR Directory Structure for an Oracle Database Instance

Alert Log

Each database has an alert log, which is an XML file containing a chronological log of database messages and errors. The alert log contents include the following:

All internal errors (ORA-600), block corruption errors (ORA-1578), and deadlock errors (ORA-60)

Administrative operations such as DDL statements and the SQL*Plus commands STARTUP, SHUTDOWN, ARCHIVE LOG, and RECOVER

Several messages and errors relating to the functions of shared server and dispatcher processes

Errors during the automatic refresh of a materialized view

Oracle Database uses the alert log as an alternative to displaying information in the Enterprise Manager GUI. If an administrative operation is successful, then Oracle Database writes a message to the alert log as "completed" along with a time stamp.

Oracle Database creates an alert log in the alert subdirectory when you first start a database instance, even if no database has been created yet. The following example shows a portion of a text-only alert log:

Trace Files

A trace file is an administrative file that contain diagnostic data used to investigate problems. Also, trace files can provide guidance for tuning applications or an instance, as explained in

Types of Trace Files

Each server and background process can periodically write to an associated trace file. The files information on the process environment, status, activities, and errors.

The SQL trace facility also creates trace files, which provide performance information on individual SQL statements. To enable tracing for a client identifier, service, module, action, session, instance, or database, you must execute the appropriate procedures in the DBMS_MONITOR package or use Oracle Enterprise Manager.

A dump is a special type of trace file. Whereas a trace tends to be continuous output of diagnostic data, a dump is typically a one-time output of diagnostic data in response to an event (such as an incident). When an incident occurs, the database writes one or more dumps to the incident directory created for the incident. Incident dumps also contain the incident number in the file name.

Locations of Trace Files

ADR stores trace files in the trace subdirectory, as shown in. Trace file names are platform-dependent and use the extension .trc.

Typically, database background process trace file names contain the Oracle SID, the background process name, and the operating system process number. An example of a trace file for the RECO process is mytest_reco_10355.trc.

Server process trace file names contain the Oracle SID, the string ora, and the operating system process number. An example of a server process trace file name is mytest_ora_10304.trc.

Sometimes trace files have corresponding trace map (.trm) files. These files contain structural information about trace files and are used for searching and navigation.

Password file – specifies which *special* users are authenticated to startup/shut down an Oracle Instance.

Memory Management and Memory Structures

 

Oracle Database Memory Management

 Memory management - focus is to maintain optimal sizes for memory structures.

        Memory is managed based on memory-related initialization parameters.

        These values are stored in the init.ora file for each database.

 Three basic options for memory management are as follows:

        Automatic memory management:

o   DBA specifies the target size for instance memory. o   The database instance automatically tunes to the target memory size.o   Database redistributes memory as needed between the SGA and the

instance PGA.

         Automatic shared memory management:

o   This management mode is partially automated. o   DBA specifies the target size for the SGA.o   DBA can optionally set an aggregate target size for the PGA or

managing PGA work areas individually.

         Manual memory management:

o   Instead of setting the total memory size, the DBA sets many initialization parameters to manage components of the SGA and instance PGA individually.

 

If you create a database with Database Configuration Assistant (DBCA) and choose the basic installation option, then automatic memory management is the default.

 

The memory structures include three areas of memory:

        System Global Area (SGA) – this is allocated when an Oracle Instance starts up.

        Program Global Area (PGA) – this is allocated when a Server Process starts up.

        User Global Area (UGA) – this is allocated when a user connects to create a session.

System Global Area

 The SGA is a read/write memory area that stores information shared by all database processes and by all users of the database (sometimes it is called the Shared Global Area).

o   This information includes both organizational data and control information used by the Oracle Server.

o   The SGA is allocated in memory and virtual memory.

o   The size of the SGA can be established by a DBA by assigning a value to the parameter SGA_MAX_SIZE in the parameter file—this is an optional parameter.

 The SGA is allocated when an Oracle instance (database) is started up based on values specified in the initialization parameter file (either PFILE or SPFILE).

 The SGA has the following mandatory memory structures:

        Database Buffer Cache

        Redo Log Buffer

        Java Pool

        Streams Pool

        Shared Pool – includes two components:

o   Library Cache

o   Data Dictionary Cache

        Other structures (for example, lock and latch management, statistical data)

 Additional optional memory structures in the SGA include:

        Large Pool

 The SHOW SGA SQL command will show you the SGA memory allocations.

        This is a recent clip of the SGA for the DBORCL database at SIUE.

        In order to execute SHOW SGA you must be connected with the special privilege SYSDBA (which is only available to user accounts that are members of the DBA Linux group).

 

SQL> connect / as sysdba

Connected.

SQL> show sga

 

Total System Global Area 1610612736 bytes

Fixed Size 2084296 bytes

Variable Size 1006633528 bytes

Database Buffers 587202560 bytes

Redo Buffers 14692352 bytes

Early versions of Oracle used a Static SGA. This meant that if modifications to memory management were required, the database had to be shutdown, modifications were made to the init.ora parameter file, and then the database had to be restarted.

 Oracle 11g uses a Dynamic SGA. Memory configurations for the system global area can be made without shutting down the database instance. The DBA can resize the Database Buffer Cache and Shared Pool dynamically.

 Several initialization parameters are set that affect the amount of random access memory dedicated to the SGA of an Oracle Instance. These are:

         SGA_MAX_SIZE: This optional parameter is used to set a limit on the amount of virtual memory allocated to the SGA – a typical setting might be 1 GB; however, if the value for SGA_MAX_SIZE in the initialization parameter file or server parameter file is less than the sum the memory allocated for all components, either explicitly in the parameter file or by default, at the time the instance is initialized, then the database ignores the setting for SGA_MAX_SIZE. For optimal performance, the entire SGA should fit in real memory to eliminate paging to/from disk by the operating system.

Buffer Caches

 A number of buffer caches are maintained in memory in order to improve system response time.

 Database Buffer Cache

 

The Database Buffer Cache is a fairly large memory object that stores the actual data blocks that are retrieved from datafiles by system queries and other data manipulation language commands.

 The purpose is to optimize physical input/output of data.

 When Database Smart Flash Cache (flash cache) is enabled, part of the buffer cache can reside in the flash cache.

        This buffer cache extension is stored on a flash disk device, which is a solid state storage device that uses flash memory.

        The database can improve performance by caching buffers in flash memory instead of reading from magnetic disk.

        Database Smart Flash Cache is available only in Solaris and Oracle Enterprise Linux.

A query causes a Server Process to look for data.

        The first look is in the Database Buffer Cache to determine if the requested information happens to already be located in memory – thus the information would not need to be retrieved from disk and this would speed up performance.

        If the information is not in the Database Buffer Cache, the Server Process retrieves the information from disk and stores it to the cache.

        Keep in mind that information read from disk is read a block at a time, NOT a row at a time, because a database block is the smallest addressable storage space on disk.

 Database blocks are kept in the Database Buffer Cache according to a Least Recently Used (LRU) algorithm and are aged out of memory if a buffer cache block is not used in order to provide space for the insertion of newly needed database blocks.

 There are three buffer states:

        Unused - a buffer is available for use - it has never been used or is currently unused.

        Clean - a buffer that was used earlier - the data has been written to disk.

        Dirty - a buffer that has modified data that has not been written to disk.

 Each buffer has one of two access modes:

        Pinned - a buffer is pinned so it does not age out of memory.

        Free (unpinned).

 The buffers in the cache are organized in two lists:

        the write list and,

        the least recently used (LRU) list.

 The write list (also called a write queue) holds dirty buffers – these are buffers that hold that data that has been modified, but the blocks have not been written back to disk.

 The LRU list holds unused, free clean buffers, pinned buffers, and free dirty buffers that have not yet been moved to the write list. Free clean buffers do not contain any useful data and are available for use. Pinned buffers are currently being accessed.

 When an Oracle process accesses a buffer, the process moves the buffer to the most recently used (MRU) end of the LRU list – this causes dirty buffers to age toward the LRU end of the LRU list.

 When an Oracle user process needs a data row, it searches for the data in the database buffer cache because memory can be searched more quickly than hard disk can be accessed. If the data row is already in the cache (a cache hit), the process reads the data from memory; otherwise a cache miss occurs and data must be read from hard disk into the database buffer cache.

 Before reading a data block into the cache, the process must first find a free buffer. The process searches the LRU list, starting at the LRU end of the list. The search continues until a free buffer is found or until the search reaches the threshold limit of buffers.

 Each time a user process finds a dirty buffer as it searches the LRU, that buffer is moved to the write list and the search for a free buffer continues.

When a user process finds a free buffer, it reads the data block from disk into the buffer and moves the buffer to the MRU end of the LRU list.

 If an Oracle user process searches the threshold limit of buffers without finding a free buffer, the process stops searching the LRU list and signals the DBWn background process to write some of the dirty buffers to disk. This frees up some buffers.

 Database Buffer Cache Block Size

 The block size for a database is set when a database is created and is determined by the init.ora parameter file parameter named DB_BLOCK_SIZE.

        Typical block sizes are 2KB, 4KB, 8KB, 16KB, and 32KB.

        The size of blocks in the Database Buffer Cache matches the block size for the database.

        The DBORCL database uses an 8KB block size.

        This figure shows that the use of non-standard block sizes results in multiple database buffer cache memory allocations.

 

 

 

Because tablespaces that store oracle tables can use different (non-standard) block sizes, there can be more than one Database Buffer Cache allocated to match block sizes in the cache with the block sizes in the non-standard tablespaces.

 The size of the Database Buffer Caches can be controlled by the parameters DB_CACHE_SIZE and DB_nK_CACHE_SIZE to dynamically change the memory allocated to the caches without restarting the Oracle instance.

 You can dynamically change the size of the Database Buffer Cache with the ALTER SYSTEM command like the one shown here:

 ALTER SYSTEM SET DB_CACHE_SIZE = 96M;

 You can have the Oracle Server gather statistics about the Database Buffer Cache to help you size it to achieve an optimal workload for the memory allocation. This information is displayed from the V$DB_CACHE_ADVICE view. In order for statistics to be gathered, you can dynamically alter the system by using the ALTER SYSTEM SET DB_CACHE_ADVICE (OFF, ON, READY) command. However, gathering statistics on system performance always incurs some overhead that will slow down system performance.

 SQL> ALTER SYSTEM SET db_cache_advice = ON;

SQL> SELECT name, block_size, advice_status FROM v$db_cache_advice;

 

NAME BLOCK_SIZE ADV

-------------------- ---------- ---

DEFAULT 8192 ON

SQL> ALTER SYSTEM SET db_cache_advice = OFF;

 KEEP Buffer Pool

 This pool retains blocks in memory (data from tables) that are likely to be reused throughout daily processing. An example might be a table containing user names and passwords or a validation table of some type.

 The DB_KEEP_CACHE_SIZE parameter sizes the KEEP Buffer Pool.

 RECYCLE Buffer Pool

 This pool is used to store table data that is unlikely to be reused throughout daily processing – thus the data blocks are quickly removed from memory when not needed.

 The DB_RECYCLE_CACHE_SIZE parameter sizes the Recycle Buffer Pool.

Redo Log Buffer

 

The Redo Log Buffer memory object stores images of all changes made to database blocks.

        Database blocks typically store several table rows of organizational data. This means that if a single column value from one row in a block is changed, the block image is stored. Changes include INSERT, UPDATE, DELETE, CREATE, ALTER, or DROP.

        LGWR writes redo sequentially to disk while DBWn performs scattered writes of data blocks to disk.

o   Scattered writes tend to be much slower than sequential writes. o   Because LGWR enable users to avoid waiting for DBWn to complete

its slow writes, the database delivers better performance.

 The Redo Log Buffer as a circular buffer that is reused over and over. As the buffer fills up, copies of the images are stored to the Redo Log Files that are covered in more detail in a later module.

 Large Pool

 The Large Pool is an optional memory structure that primarily relieves the memory burden placed on the Shared Pool. The Large Pool is used for the following tasks if it is allocated:

        Allocating space for session memory requirements from the User Global Area where a Shared Server is in use.

        Transactions that interact with more than one database, e.g., a distributed database scenario.

        Backup and restore operations by the Recovery Manager (RMAN) process.

o   RMAN uses this only if the BACKUP_DISK_IO = n and BACKUP_TAPE_IO_SLAVE = TRUE parameters are set.

o   If the Large Pool is too small, memory allocation for backup will fail and memory will be allocated from the Shared Pool.

        Parallel execution message buffers for parallel server operations. The PARALLEL_AUTOMATIC_TUNING = TRUE parameter must be set.

 The Large Pool size is set with the LARGE_POOL_SIZE parameter – this is not a dynamic parameter. It does not use an LRU list to manage memory.

 

 

Java Pool

 The Java Pool is an optional memory object, but is required if the database has Oracle Java installed and in use for Oracle JVM (Java Virtual Machine).

        The size is set with the JAVA_POOL_SIZE parameter that defaults to 24MB.

        The Java Pool is used for memory allocation to parse Java commands and to store data associated with Java commands.

        Storing Java code and data in the Java Pool is analogous to SQL and PL/SQL code cached in the Shared Pool.

 Streams Pool

 This pool stores data and control structures to support the Oracle Streams feature of Oracle Enterprise Edition.

        Oracle Steams manages sharing of data and events in a distributed environment.

        It is sized with the parameter STREAMS_POOL_SIZE.

        If STEAMS_POOL_SIZE is not set or is zero, the size of the pool grows dynamically

Shared Pool

 

 

 

The Shared Pool is a memory structure that is shared by all system users.

        It caches various types of program data. For example, the shared pool stores parsed SQL, PL/SQL code, system parameters, and data dictionary information.

        The shared pool is involved in almost every operation that occurs in the database. For example, if a user executes a SQL statement, then Oracle Database accesses the shared pool.

        It consists of both fixed and variable structures.

        The variable component grows and shrinks depending on the demands placed on memory size by system users and application programs.

 Memory can be allocated to the Shared Pool by the parameter SHARED_POOL_SIZE in the parameter file. The default value of this parameter is 8MB on 32-bit platforms and 64MB on 64-bit platforms. Increasing the value of this parameter increases the amount of memory reserved for the shared pool.

 

You can alter the size of the shared pool dynamically with the ALTER SYSTEM SET command. An example command is shown in the figure below. You must keep in mind that the total memory allocated to the SGA is set by the SGA_TARGET parameter (and may also be limited by the SGA_MAX_SIZE if it is set), and since the Shared Pool is part of the SGA, you cannot exceed the maximum size of the SGA. It is recommended to let Oracle optimize the Shared Pool size.

 The Shared Pool stores the most recently executed SQL statements and used data definitions. This is because some system users and application programs will tend to execute the same SQL statements often. Saving this information in memory can improve system performance.

 The Shared Pool includes several cache areas described below.

 Library Cache

 Memory is allocated to the Library Cache whenever an SQL statement is parsed or a program unit is called. This enables storage of the most recently used SQL and PL/SQL statements.

 If the Library Cache is too small, the Library Cache must purge statement definitions in order to have space to load new SQL and PL/SQL statements. Actual management of this memory structure is through a Least-Recently-Used (LRU) algorithm. This means that the SQL and PL/SQL statements that are oldest and least recently used are purged when more storage space is needed.

 The Library Cache is composed of two memory subcomponents:

        Shared SQL: This stores/shares the execution plan and parse tree for SQL statements, as well as PL/SQL statements such as functions, packages, and triggers. If a system user executes an identical statement, then the statement does not have to be parsed again in order to execute the statement.

        Private SQL Area: With a shared server, each session issuing a SQL statement has a private SQL area in its PGA.

o   Each user that submits the same statement has a private SQL area pointing to the same shared SQL area.

o   Many private SQL areas in separate PGAs can be associated with the same shared SQL area.

o   This figure depicts two different client processes issuing the same SQL statement – the parsed solution is already in the Shared SQL Area.

 

 

 

Data Dictionary Cache

 The Data Dictionary Cache is a memory structure that caches data dictionary information that has been recently used.

        This cache is necessary because the data dictionary is accessed so often.

        Information accessed includes user account information, datafile names, table descriptions, user privileges, and other information.

 The database server manages the size of the Data Dictionary Cache internally and the size depends on the size of the Shared Pool in which the Data Dictionary Cache resides. If the size is too small, then the data dictionary tables that reside on disk must be queried often for information and this will slow down performance.

 

Server Result Cache

 The Server Result Cache holds result sets and not data blocks. The server result cache contains the SQL query result cache and PL/SQL function result cache, which share the same infrastructure.

 SQL Query Result Cache

 This cache stores the results of queries and query fragments.

        Using the cache results for future queries tends to improve performance.

        For example, suppose an application runs the same SELECT statement repeatedly. If the results are cached, then the database returns them immediately.

        In this way, the database avoids the expensive operation of rereading blocks and recomputing results.

 PL/SQL Function Result Cache

The PL/SQL Function Result Cache stores function result sets.

        Without caching, 1000 calls of a function at 1 second per call would take 1000 seconds.

        With caching, 1000 function calls with the same inputs could take 1 second total.

        Good candidates for result caching are frequently invoked functions that depend on relatively static data.

        PL/SQL function code can specify that results be cached.

Program Global Area (PGA)

 A PGA is:

        a nonshared memory region that contains data and control information exclusively for use by an Oracle process.

        A PGA is created by Oracle Database when an Oracle process is started.

        One PGA exists for each Server Process and each Background Process. It stores data and control information for a single Server Process or a single Background Process.

        It is allocated when a process is created and the memory is scavenged by the operating system when the process terminates. This is NOT a shared part of memory – one PGA to each process only.

        The collection of individual PGAs is the total instance PGA, or instance PGA.

        Database initialization parameters set the size of the instance PGA, not individual PGAs.

 The Program Global Area is also termed the Process Global Area (PGA) and is a part of memory allocated that is outside of the Oracle Instance.

 

 

 

 

The content of the PGA varies, but as shown in the figure above, generally includes the following:

 

        Private SQL Area: Stores information for a parsed SQL statement – stores bind variable values and runtime memory allocations. A user session issuing SQL statements has a Private SQL Area that may be associated with a Shared SQL Area if the same SQL statement is being executed by more than one system user. This often happens in OLTP environments where many users are executing and using the same application program.

o   Dedicated Server environment – the Private SQL Area is located in the Program Global Area.

o   Shared Server environment – the Private SQL Area is located in the System Global Area.

 

        Session Memory: Memory that holds session variables and other session information.

 

        SQL Work Areas: Memory allocated for sort, hash-join, bitmap merge, and bitmap create types of operations.

o   Oracle 9i and later versions enable automatic sizing of the SQL Work Areas by setting the WORKAREA_SIZE_POLICY = AUTO parameter (this is the default!) and PGA_AGGREGATE_TARGET = n (where n is some amount of memory established by the DBA). However, the DBA can let the Oracle DBMS determine the appropriate amount of memory.

 

 

User Global Area

The User Global Area is session memory.

 

 

A session that loads a PL/SQL package into memory has the package state stored to the UGA. The package state is the set of values stored in all the package variables at a specific time. The state changes as program code the variables. By default, package variables are unique to and persist for the life of the session.

The OLAP page pool is also stored in the UGA. This pool manages OLAP data pages, which are equivalent to data blocks. The page pool is allocated at the start of an OLAP session and released at the end of the session. An OLAP session opens automatically whenever a user queries a dimensional object such as a cube.

Note: Oracle OLAP is a multidimensional analytic engine embedded in Oracle Database 11g. Oracle OLAP cubes deliver sophisticated calculations using simple SQL queries - producing results with speed of thought response times.

The UGA must be available to a database session for the life of the session. For this reason, the UGA cannot be stored in the PGA when using a shared server connection because the PGA is specific to a single process. Therefore, the UGA is stored in the SGA when using shared server connections, enabling any shared server process access to it. When using a dedicated server connection, the UGA is stored in the PGA.

Automatic Shared Memory Management

Prior to Oracle 10G, a DBA had to manually specify SGA Component sizes through the initialization parameters, such as SHARED_POOL_SIZE, DB_CACHE_SIZE, JAVA_POOL_SIZE, and LARGE_POOL_SIZE parameters.

 Automatic Shared Memory Management enables a DBA to specify the total SGA memory available through the SGA_TARGET initialization parameter. The Oracle Database automatically distributes this memory among various subcomponents to ensure most effective memory utilization.The DBORCL database SGA_TARGET is set in the initDBORCL.ora file:sga_target=1610612736

With automatic SGA memory management, the different SGA components are flexibly sized to adapt to the SGA available.

Setting a single parameter simplifies the administration task – the DBA only specifies the amount of SGA memory available to an instance – the DBA can forget about the sizes of individual components. No out of memory errors are generated unless the system has actually run out of memory. No manual tuning effort is needed.

The SGA_TARGET initialization parameter reflects the total size of the SGA and includes memory for the following components:

Fixed SGA and other internal allocations needed by the Oracle Database instance

The log buffer The shared pool The Java pool The buffer cache The keep and recycle buffer caches (if specified) Nonstandard block size buffer caches (if specified) The Streams Pool

 If SGA_TARGET is set to a value greater than SGA_MAX_SIZE at startup, then the SGA_MAX_SIZE value is bumped up to accommodate SGA_TARGET.

When you set a value for SGA_TARGET, Oracle Database 11g automatically sizes the most commonly configured components, including:

The shared pool (for SQL and PL/SQL execution) The Java pool (for Java execution state) The large pool (for large allocations such as RMAN backup buffers) The buffer cache

 There are a few SGA components whose sizes are not automatically adjusted. The DBA must specify the sizes of these components explicitly, if they are needed by an application. Such components are:

Keep/Recycle buffer caches (controlled by DB_KEEP_CACHE_SIZE and DB_RECYCLE_CACHE_SIZE)

Additional buffer caches for non-standard block sizes (controlled by DB_nK_CACHE_SIZE, n = {2, 4, 8, 16, 32})

Streams Pool (controlled by the new parameter STREAMS_POOL_SIZE)

Processes

 You need to understand three different types of Processes:

        User / client Process: Starts when a database user requests to connect to an Oracle Server.

        Server Process: Establishes the Connection to an Oracle Instance when a User Process requests connection – makes the connection for the User Process.

        Background Processes: These start when an Oracle Instance is started up.

This generates a User Process (a memory object) that generates programmatic calls through your user interface (SQLPlus, Integrated Developer Suite, or application program) that creates a session and causes the generation of a Server Process that is either dedicated or shared.

 

A Server Process is the go-between for a Client Process and the Oracle Instance.

        Dedicated Server environment – there is a single Server Process to serve each Client Process.

        Shared Server environment – a Server Process can serve several User Processes, although with some performance reduction.

        Allocation of server process in a dedicated environment versus a shared environment is covered in further detail in the

Background Processes

 As is shown here, there are both mandatory, optional, and slave background processes that are started whenever an Oracle Instance starts up. These background processes serve all system users. We will cover mandatory process in detail.

  Mandatory Background Processes

        Process Monitor Process (PMON)        System Monitor Process (SMON)        Database Writer Process (DBWn)        Log Writer Process (LGWR)        Checkpoint Process (CKPT)        Manageability Monitor Processes (MMON and MMNL)        Recover Process (RECO)

Optional Processes

        Archiver Process (ARCn)        Coordinator Job Queue (CJQ0)        Dispatcher (number “nnn”) (Dnnn)        Others

 

This query will display all background processes running to serve a database:

 

SELECT PNAME FROM V$PROCESS WHERE PNAME IS NOT NULL ORDER BY PNAME;PMON

 The Process Monitor (PMON) monitors other background processes.

        It is a cleanup type of process that cleans up after failed processes.

        Examples include the dropping of a user connection due to a network failure or the abnormal termination (ABEND) of a user application program.

        It cleans up the database buffer cache and releases resources that were used by a failed user process.

PMON also checks the dispatcher & server processes and restarts them if they have failed.

PMON wakes up every 3 seconds to perform housekeeping activities.

In RAC, PMON’s role as service registration agent is particularly important.

        It does the tasks shown in the figure below.

 

 

 SMON

 The System Monitor (SMON) does system-level cleanup duties.

        It is responsible for instance recovery by applying entries in the online redo log files to the datafiles.

        Other processes can call SMON when it is needed.

        It also performs other activities as outlined in the figure shown below.

 

 

If an Oracle Instance fails, all information in memory not written to disk is lost. SMON is responsible for recovering the instance when the database is started up again. It does the following:

        Rolls forward to recover data that was recorded in a Redo Log File, but that had not yet been recorded to a datafile by DBWn. SMON reads the Redo Log Files and applies the changes to the data blocks. This recovers all transactions that were committed because these were written to the Redo Log Files prior to system failure.

        Opens the database to allow system users to logon.

        Rolls back uncommitted transactions.

 SMON also does limited space management. It combines (coalesces) adjacent areas of free space in the database's datafiles for tablespaces that are dictionary managed.

 It also deallocates temporary segments to create free space in the datafiles.

 

 DBWn (also called DBWR in earlier Oracle Versions)

 

The Database Writer writes modified blocks from the database buffer cache to the datafiles.

        One database writer process (DBW0) is sufficient for most systems.

        A DBA can configure up to 20 DBWn processes (DBW0 through DBW9 and DBWa through DBWj) in order to improve write performance for a system that modifies data heavily.

        The initialization parameter DB_WRITER_PROCESSES specifies the number of DBWn processes. 

 The purpose of DBWn is to improve system performance by caching writes of database blocks from the Database Buffer Cache back to datafiles.

        Blocks that have been modified and that need to be written back to disk are termed "dirty blocks."

        The DBWn also ensures that there are enough free buffers in the Database Buffer Cache to service Server Processes that may be reading data from datafiles into the Database Buffer Cache.

        Performance improves because by delaying writing changed database blocks back to disk, a Server Process may find the data that is needed to meet a User Process request already residing in memory! 

        DBWn writes to datafiles when one of these events occurs that is illustrated in the figure below.

 

 

 

LGWR

 

The Log Writer (LGWR) writes contents from the Redo Log Buffer to the Redo Log File that is in use.

        These are sequential writes since the Redo Log Files record database modifications based on the actual time that the modification takes place.

        LGWR actually writes before the DBWn writes and only confirms that a COMMIT operation has succeeded when the Redo Log Buffer contents are successfully written to disk.

        LGWR can also call the DBWn to write contents of the Database Buffer Cache to disk.

        The LGWR writes according to the events illustrated in the figure shown below.

Whenever checkpoint event occurs.

 

CKPT

 The Checkpoint (CPT) process writes information to update the database control files and headers of datafiles.

        A checkpoint identifies a point in time with regard to the Redo Log Files where instance recovery is to begin should it be necessary.

        It can tell DBWn to write blocks to disk.         A checkpoint is taken at a minimum, once every three seconds.

Checkpoint is a background process which triggers the checkpoint event, to synchronize all database files with thecheckpoint information. It ensures data consistency and faster database recovery in case of a crash.

When checkpoint occurred it will invoke the DBWn and updates the SCN block of the all datafiles and the control file with the current SCN. This is done by LGWR. This SCN is called checkpoint SCN. Checkpoint event can be occurred in following conditions:o Whenever database buffer cache filled up.o Whenever times out (3seconds until 9i, 1second from 10g).o Log switch occurred.o Whenever manual log switch is done.SQL> ALTER SYSTEM SWITCH LOGFILE;o Manual checkpoint.SQL> ALTER SYSTEM CHECKPOINT;o Graceful shutdown of the database.o Whenever BEGIN BACKUP command is issued.

o When the time specified by the initialization parameter LOG_CHECKPOINT_TIMEOUT (in seconds), exists between the incremental checkpoint and the tail of the log.o When the number of OS blocks specified by the initialization parameter LOG_CHECKPOINT_INTERVAL, exists between the incremental checkpoint and the tail of the log.o The number of buffers specified by the initialization parameter FAST_START_IO_TARGET required to perform roll-forward is reached.o Oracle 9i onwards, the time specified by the initialization parameter FAST_START_MTTR_TARGET (in seconds) is reached and specifies the time required for a crash recovery. The parameter FAST_START_MTTR_TARGET replaces LOG_CHECKPOINT_INTERVAL and FAST_START_IO_TARGET, but these parameters can still be used.

 

 

Think of a checkpoint record as a starting point for recovery. DBWn will have completed writing all buffers from the Database Buffer Cache to disk prior to the checkpoint, thus those records will not require recovery. This does the following:

        Ensures modified data blocks in memory are regularly written to disk – CKPT can call the DBWn process in order to ensure this and does so when writing a checkpoint record.

        Reduces Instance Recovery time by minimizing the amount of work needed for recovery since only Redo Log File entries processed since the last checkpoint require recovery.

        Causes all committed data to be written to datafiles during database shutdown.

 

 

If a Redo Log File fills up and a switch is made to a new Redo Log File (this is covered in more detail in a later module), the CKPT process also writes checkpoint information into the headers of the datafiles.

 

Checkpoint information written to control files includes the system change number (the SCN is a number stored in the control file and in the headers of the database files that are used to ensure that all files in the system are synchronized), location of which Redo Log File is to be used for recovery, and other information.

 

CKPT does not write data blocks or redo blocks to disk – it calls DBWn and LGWR as necessary.

 

MMON and MMNL

The Manageability Monitor Process (MMNO) performs tasks related to the Automatic Workload Repository (AWR) – a repository of statistical data in the SYSAUX tablespace (see figure below) – for example, MMON writes when a metric violates its threshold value, taking snapshots, and capturing statistics value for recently modified SQL objects.

 

 

The Manageability Monitor Lite Process (MMNL) writes statistics from the Active Session History (ASH) buffer in the SGA to disk. MMNL writes to disk when the ASH buffer is full.

The information stored by these processes is used for performance tuning – we survey performance tuning in a later module.

Memory Manager (maximum 1) MMANMMAN dynamically adjust the sizes of the SGA components like buffer cache, large pool, shared pool and java pool and serves as SGA memory broker. It is a new process added to Oracle 10g as part of automatic shared memory management.

 

RECO

 

The Recoverer Process (RECO) is used to resolve failures of distributed transactions in a distributed database.

        Consider a database that is distributed on two servers – one in St. Louis and one in Chicago.

        Further, the database may be distributed on servers of two different operating systems, e.g. LINUX and Windows.

        The RECO process of a node automatically connects to other databases involved in an in-doubt distributed transaction.

        When RECO reestablishes a connection between the databases, it automatically resolves all in-doubt transactions, removing from each database's pending transaction table any rows that correspond to the resolved transactions.

 Optional Background Processes

 Optional Background Process Definition:

        ARCn: (maximum 10) ARC0-ARC9 Archiver – One or more archiver processes copy the online redo log files to archival storage when they are full or a log switch occurs.

 ARCn

While the Archiver (ARCn) is an optional background process, we cover it in more detail because it is almost always used for production systems storing mission critical information.

        The ARCn process must be used to recover from loss of a physical disk drive for systems that are "busy" with lots of transactions being completed.

        It performs the tasks listed below.

 

When a Redo Log File fills up, Oracle switches to the next Redo Log File.

        The DBA creates several of these and the details of creating them are covered in a later module.

        If all Redo Log Files fill up, then Oracle switches back to the first one and uses them in a round-robin fashion by overwriting ones that have already been used.

        Overwritten Redo Log Files have information that, once overwritten, is lost forever.

 

ARCHIVELOG Mode:

        If ARCn is in what is termed ARCHIVELOG mode, then as the Redo Log Files fill up, they are individually written to Archived Redo Log Files.

        LGWR does not overwrite a Redo Log File until archiving has completed.

        Committed data is not lost forever and can be recovered in the event of a disk failure.

        Only the contents of the SGA will be lost if an Instance fails.

 

In NOARCHIVELOG Mode:

        The Redo Log Files are overwritten and not archived.         Recovery can only be made to the last full backup of the database files.         All committed transactions after the last full backup are lost, and you can

see that this could cost the firm a lot of $$$.

 

When running in ARCHIVELOG mode, the DBA is responsible to ensure that the Archived Redo Log Files do not consume all available disk space! Usually after two complete backups are made, any Archived Redo Log Files for prior backups are deleted.

        CJQ0: Coordinator Job Queue – This is the coordinator of job queue processes for an instance. It monitors the JOB$ table (table of jobs in the job queue) and starts job queue processes (Jnnn) as needed to execute jobs The Jnnn processes execute job requests created by the DBMS_JOBS package.

Job queue processes carry out batch processing. All scheduled jobs are executed by these processes. The initialization parameter JOB_QUEUE_PROCESSES specifies the maximum job processes that can be run concurrently. These processes will be useful in refreshing materialized views.

This is the Oracle’s dynamic job queue coordinator. It periodically selects jobs (from JOB$) that need to be run, scheduled by the Oracle job queue. The coordinator process dynamically spawns job queue slave processes (J000-J999) to run the jobs. These jobs could be PL/SQL statements or procedures on an Oracle instance.

CQJ0 - Job queue controller process wakes up periodically and checks the job log. If a job is due, it spawns Jnnnn processes to handle jobs.

From Oracle 11g release2, DBMS_JOB and DBMS_SCHEDULER work without setting JOB_QUEUE_PROCESSES. Prior to11gR2 the default value is 0, and from 11gR2 the default value is 1000.

Shared Server Processes (maximum 1000) SnnnIntended for multi threaded server (MTS) setups. These processes pickup requests from the call request queue, process them and then return the results to a result queue. These user processes also handle disk reads from database datafiles into the database block buffers. The number of shared server processes to be created at instance startup can be specified using the initialization parameter SHARED_SERVERS. Maximum shared server processes can be specified by MAX_SHARED_SERVERS.

        Dnnn: Dispatcher number "nnn", for example, D000 would be the first dispatcher process – Dispatchers are optional background processes, present only when the shared server configuration is used. Shared server is discussed in your readings on the topic "Configuring Oracle for the Shared Server". 

Intended for multi threaded server (MTS) setups. Dispatcher processes listen to and receive requests from connected sessions and places them in the request queue for further processing. Dispatcher processes also pickup outgoing responses from the result queue and transmit them back to the clients. Dnnn are mediators between the client processes and the shared server processes. The maximum number of dispatcher process can be specified using the initialization parameter MAX_DISPATCHERS.

Trace Writer (maximum 1) TRWRTrace writer writes trace files from an Oracle internal tracing facility.

        FBDA: Flashback Data Archiver Process – This archives historical rows of tracked tables into Flashback Data Archives. When a transaction containing DML on a tracked table commits, this process stores the pre-image of the rows into the Flashback Data Archive. It also keeps metadata on the current rows. FBDA automatically manages the flashback data archive for space, organization, and retention

 

 Slave Processes

 Slave processes are background processes that perform work on behalf of other processes.

 Innn: I/O slave processes -- simulate asynchronous I/O for systems and devices that do not support it. In asynchronous I/O, there is no timing requirement for transmission, enabling other processes to start before the transmission has finished.

        For example, assume that an application writes 1000 blocks to a disk on an operating system that does not support asynchronous I/O.

        Each write occurs sequentially and waits for a confirmation that the write was successful.

        With asynchronous disk, the application can write the blocks in bulk and perform other work while waiting for a response from the operating system that all blocks were written.

 

Parallel Query Slaves -- In parallel execution or parallel processing, multiple processes work together simultaneously to run a single SQL statement.

        By dividing the work among multiple processes, Oracle Database can run the statement more quickly.

        For example, four processes handle four different quarters in a year instead of one process handling all four quarters by itself.

        Parallel execution reduces response time for data-intensive operations on large databases such as data warehouses. Symmetric multiprocessing (SMP) and clustered system gain the largest performance benefits from parallel execution because statement processing can be split up among multiple CPUs. Parallel execution can also benefit certain types of OLTP and hybrid systems.

SQL Statement Processing

 

SQL Statements are processed differently depending on whether the statement is a query, data manipulation language (DML) to update, insert, or delete a row, or data definition language (DDL) to write information to the data dictionary.

 

 

Processing a query:

        Parse:

Check syntax, object names, and privileges. Search for identical statement in the Shared SQL Area.

Create and store execution plan. Lock objects used during parse.

        Bind: Obtains values for variables.

        Execute: Process statement.

        Fetch: Return rows to user process.

 

Processing a DML statement:

        Parse: Same as the parse phase used for processing a query.

        Bind: Same as the bind phase used for processing a query.

        Execute:

o   If the data and undo blocks are not already in the Database Buffer Cache, the server process reads them from the datafiles into the Database Buffer Cache.

o   The server process places locks on the rows that are to be modified. The undo block is used to store the before image of the data, so that the DML statements can be rolled back if necessary.

o   The data blocks record the new values of the data.

o   The server process records the before image to the undo block and updates the data block. Both of these changes are made in the Database Buffer Cache. Any changed blocks in the Database Buffer Cache are marked as dirty buffers. That is, buffers that are not the same as the corresponding blocks on the disk.

o   The processing of a DELETE or INSERT command uses similar steps. The before image for a DELETE contains the column values in the deleted row, and the before image of an INSERT contains the row location information.

 

Processing a DDL statement:

        The execution of DDL (Data Definition Language) statements differs from the execution of DML (Data Manipulation Language) statements and queries, because the success of a DDL statement requires write access to the data dictionary.

        For these statements, parsing actually includes parsing, data dictionary lookup, and execution. Transaction management, session management, and system management SQL statements are processed using the parse and execute stages. To re-execute them, simply perform another execute