databasesystemer e2002lene pries-hejedata structure, storage and processing architectures...

Lene Pries-Heje Data Structure, Storage and Processing Architectures

Databasesystemer E2002

Databasesystemer

Data Structure, Storage and Processing Architectures



Learning objectives

• Be able to explain what a database architecture is and what goals the design strives to achieve.

• Know different data storage structures and storage devices, and when to use them.

• Know the 4 basic architectures, and the differences between them.



Database Architectures and Implementations

We shape our buildings: thereafter they shape us

Winston Churchill



Database Architectures

• Database architecture is a design for the storage and processing of Data.



Goals

• An architecture should– Respond to queries in a timely manner– Minimize the cost of processing data– Minimize the cost of storing data– Minimize the cost of data delivery

• These objectives can be conflicting



ANSI SPARC

Storage view

Conceptuallevel

Internallevel

Externallevel

Database designer's view

Userview

Userview

Userview



Data Structures

• The goal is to minimize disk accesses• Disks are relatively slow compared to main

memory– Writing a letter compared to a telephone call

• Disks are a bottleneck• Appropriate data structures can reduce disk

accesses



Database access

DBMSFile

managerDisk

manager

Recordrequest

Pagerequest

Readpage

command

Pageread

Pagereturned

Recordreturned



Disks

• Data stored on tracks on a surface• A disk drive can have multiple surfaces • Rotational delay

– Waiting for the physical storage location of the data to appear under the read/write head

– Around 5 msec for a magnetic disk– Set by the manufacturer

• Access arm delay– Moving the read/write head to the track on which the storage

location can be found.– Around 10 msec for a magnetic disk



How can you minimize data access times?

• Rotational delay is fixed by the manufacturer

• Access arm delay can be reduced by storing files on– The same track– The same track on each surface

• A cylinder



Clustering

• Records that are often retrieved together should be stored together

• Intra-file clustering– Records within the one file

• A sequential file

• Inter-file clustering– Records in different files

• A nation and its stocks

STOCK

*stock codefirm namestock price

stock quantitystock dividend

stock PE

NATION

*nation codenation name

exchange rate



A disk

Disk armDisk head

Arm movementRotation

CylinderTracks, bloks and sectors



Disk manager

• Manages physical I/O

• Sees the disk as a collection of pages

• Has a directory of each page on a disk

• Retrieves, replaces, and manages free pages

DISK

*diskid

PAGE

*pageid



File manager

• Manages the storage of files• Sees the disk as a collection of stored files

• Each file has a unique identifier• Each record within a file has a unique record

identifier

FILE

*fileid

RECORD

*recordid

DISK

*diskid



File manager's tasks

• Create a file

• Delete a file

• Retrieve a record from a file

• Update a record in a file

• Add a new record to a file

• Delete a record from a file



Sequential retrieval

• Consider a file of 10,000 records each occupying 1 page

• Queries that require processing all records will require 10,000 accesses– e.g., Find all items of type 'E'

• Many disk accesses are wasted if few records meet the condition



Indexing• An index is a small file that has data for one field of a

file

• Indexes reduce disk accesses

CCCCEEFNNN

ITEMTYPEINDEX

ITEMTYPE ITEMNOITEM

ITEMNAME ITEMTYPE ITEMCOLOR12345678910

EENNNCCCCF

Pocket knife–NilePocket knife–ThamesCompassGeo positioning systemMap measureHat–polar explorerHat–polar explorerBoots–snakeproofBoots–snakeproofSafari chair

BrownBrown–––RedWhiteGreenBlackKhaki



Querying with an index

• Read the index into memory

• Search the index to find records meeting the condition

• Access only those records containing required data

• Disk accesses are substantially reduced when the query involves few records



Maintaining an index

• Adding a record requires at least two disk accesses– Update the file– Update the index

• Trade-off– Faster queries– Slower maintenance



Using indexes

• Sequential processing of a portion of a file– Find all items with a type code in the range 'E' to 'K'

• Direct processing– Find all items with a type code of 'E' or 'N'

• Existence testing– Determining whether a record meeting the criteria

exists without having to retrieve it



Multiple indexes

• Find red items of type 'C'– Both indexes can be searched to identify records to

retrieveITEMCOLORINDEX

ITEMCOLOR Diskaddress

Black d9Brown d1Brown d2Green d8Khaki d10Red d6White d7– d3– d4– d5

ITEMTYPEINDEX

ITEMTYPE Diskaddress

C d6C d7C d8C d9E d1E d2F d10N d3N d4N d5



Multiple indexes

• Indexes are also called inverted lists– A file of record locations rather than data

• Trade-off– Faster retrieval– Slower maintenance



Sparse indexes• Taking advantage of the physical sequence of a file• Assume 2 records per page

• Tradeoffs– Fewer disk accesses required to read the index – Existence tests not possible

246810

ITEMNOINDEX

ITEMNO ITEMNOITEM

ITEMNAME ITEMTYPE ITEMCOLOR12345678910

EENNNCCCCF

Pocket knife–NilePocket knife–ThamesCompassGeo positioning systemMap measureHat–polar explorerHat–polar explorerBoots–snakeproofBoots–snakeproofSafari chair

BrownBrown–––RedWhiteGreenBlackKhaki

page p

page p + 1

page p + 2

page p + 3

page p + 4



B-tree

• A form of inverted list• Frequently used for relational systems• Basis of IBM’s VSAM underlying DB2• Supports sequential and direct accessing• Has two parts

– Sequence set– Index set



B-tree (B+ tree)

• Sequence set is a single level index with pointers to records

• Index set is a tree-structured index to the sequence set

1 4 5 6 19 20 26 28 29 32 33 34 40 42 46 50 54 57 63 67 82 86 93 94 95 96 98

•• • •• • •• •

•• •29

5 20

57

34 46 82 94

Index set

Sequence set



B+ tree• The combination of index set (the B-tree) and the sequence

set is called a B+ tree• The number of data values and pointers for any given node

are not restricted• Free space is set aside to permit rapid expansion of a file

• Tradeoffs– Fast retrieval when pages are packed with data values

and pointers– Slow updates when pages are packed with data values

and pointers



Hashing

• A technique for reducing disk accesses for direct access

• Avoids an index

• Number of accesses per record can be close to one

• The hash field is converted to a hash address by a hash function



Hashing

hash address = remainder after dividing SSN by 10000

417-03-4356532-67-4356891-55-4356

043-15-1893

281-27-1502

417-03-4356 532-67-4356

891-55-4356

Disk address

4356

1893

1502

SSN

Synonym chain

043-15-1893

281-27-1502

Overflow areaFile space

•

•

• •

•

}

}

}



Shortcomings of hashing

• Different hash fields convert to the same hash address– Synonyms

– Store the colliding record in an overflow area

• Long synonym chains degrade performance• There can be only one hash field• The file can no longer be processed sequentially



Linked list

• A structure for inter-file clustering

• An example of a parent/child structure

IndooroopillyRuby

•NarembeenPlum

•QueenslandDiamond

•MinnesotaGold

•GeorgiaPeach

•

Australia•

USA•• •



Linked lists

• There can be two-way pointers, forward and backward, to speed up deletion

• Each child can have a pointer to its parent



Bit map indexes

• Uses a single bit, rather than multiple bytes, to indicate the specific value of an field– Color can have only three values, so use three

bitsItemcode Color Code Disk

addressRed Green Blue A N

1001 0 0 1 0 1 d1

1002 1 0 0 1 0 d2

1003 1 0 0 1 0 d3

1004 0 1 0 1 0 d4



Bit map indexes

• A bit map index saves space and time compared to a standard index

Itemcode Color

Char(8)

Code

Char(1)

Disk address

1001 Blue N d1

1002 Red A d2

1003 Red A d3

1004 Green A d4



Join indexes

• Speed up joins by creating an index for the primary key and foreign key pairNATIONINDEX

STOCKINDEX

NATCODE Disk address NATCODE Disk addressUK d1 UK d101USA d2 UK d102

UK d103USA d104USA d105

JOIN INDEXNATIONdisk address

STOCKdisk address

d1 d101d1 d102d1 d103d2 d104d2 d105



R-trees• Used to store n-dimensional data (n>=2)

– Minimum bounding rectangle concept

A

BC

D

EX Y

D E Sequence set

Index set

A B C

X Y



R-tree searching

• Search for the object covered by the shaded region

A

BC

D

EX Y

D E Sequence set

Index set

A B C

X Y



Data storage devices

• What data storage device will be used for– On-line data

• Access speed• Capacity

– Back-up files• Security against data loss

– Archival data• Long-term storage



Key variables

• Data volume

• Data volatility

• Access speed

• Storage cost

• Medium reliability

• Legal standing of stored data



Magnetic technology

• Up to 50% of IS hardware budgets are spent on magnetic storage

• A $50 billion market

• The major form of data storage

• A mature and widely used technology

• Strong magnetic fields can erase data

• Magnetization decays with time



Fixed disks

• Sealed, permanently mounted

• Highly reliable

• Access times of 4-10 msec

• Transfer rates as high as 160 Mbytes per second

• Capacities of Gbytes to Tbytes



RAID

• Redundant arrays of inexpensive or independent drives

• Exploits economies of scale of disk manufacturing for the personal computer market

• Can also give greater security• Increases a systems fault tolerance• Not a replacement for regular backup



Mirroring

Data

Parity



Mirroring• Write

– Identical copies of a file are written to each drive in an array

• Read– Alternate pages are read simultaneously from each drive– Pages put together in memory– Access time is reduced by approximately the number of disks in the array

• Read error– Read required page from another drive

• Tradeoffs– Reduced access time– Greater security– More disk space



Striping

Data

Parity



StripingThree drive model

• Write– Half of file to first drive– Half of file to second drive– Parity bit to third drive

• Read– Portions from each drive are put together in memory

• Read error– Lost bits are reconstructed from third drive’s parity data

• Tradeoffs– Increased data security– Less storage capacity than mirroring– Not as fast as mirroring



RAID levels

• All levels, except 0, have common features

• The operating system sees a set of physical drives as one logical drive

• Data are distributed across physical drives

• Parity is used for data recovery



RAID levels• Level 0

– Data spread across multiple drives– No data recovery when a drive fails

• Level 1– Mirroring– Critical non-stop applications

• Level 3– Striping

• Level 5– A variation of striping– Parity data is spread across drives– Less capacity than level 1– Higher I/O rates than level 3



RAID 5

Data

Parity



Magnetic technology

• Removable magnetic disk

• Floppy disk

• Magnetic tape

• Magnetic tape cartridge

• Mass storage



Solid State

• Arrays of memory chips

• 10 times faster than magnetic storage

• $3 per Mbyte– Magnetic disk is about 1 cents per Mbyte

• Stock trading and video-streaming applications



Optical technology

• A more recent development

• Use a laser for reading and writing data

• High storage densities

• Low cost

• Direct access

• Long storage life

• Not susceptible to head crashes



Optical technology

Optical storage

WORMwrite once–ready many

CD-ROMwrite once–read many

Magneto-opticalwrite many–read many

DVDmultiple formats



Magneto-optical disk

• High capacity read-write medium• 3.5" disk can store up to 256 M bytes• Not as fast as fixed disk

– 10 msec access time

• Compact• Reliable• Suitable for data transfer, backup, and

archival purposes



Digital Versatile Disc (DVD)

• The same physical size as a CD-ROM but up to 28 times the capacity (i.e., 17 Gbytes)

• DVD drives are likely to have transfer rates of around 2.76 M bytes/sec and access times of 150 msec.

• DVD-ROM drive will play both audio CD's and CD-ROM's.• Read-only versions

– DVD-Video (movies)– DVD-ROM (software)– DVD-Audio (songs)

• DVD-R– Recordable (write once, read many)

• DVD-RAM– Erasable (write many, read many)



SAN

• Storage area network• Supports dynamic sharing of large amounts of

data, regardless of operating system or application• Communicates via pipelines that consist of an

interface called Fibre Channel– A high speed data connection between computer

devices

• Prices vary from $20-30,000 to 5 million



Storage life

PermanentHigh qualityNewspaper

PaperArchival quality (silver)

Medium-term filmMicrofilm

CD-R (recordable)CD-ROM (read only)

Optical disk Quarter-inch tape

VHS tapeHalf-inch tape cartridge

Half-inch reel-to-reelMagnetic tape

1 10 100 500

Storage life in years of high quality brands



Data Processing Architectures

The difficulty is in the choice

George Moore, 1900



Architecture

• ANSI/SPARC architecture was before personal computers, now there are options for where data are stored and processed



The 4 basic Architectures

Remotejob

entry

Host/terminal

Client/server

Personaldatabase

Dataprocessing

Remote

Local

Local Remote

Data storage



Remote job entry• Local storage

– Often cheaper– Maybe more secure

• Remote processing• Useful when a personal computer is:

– too slow– has insufficient memory– software is not available

• Some local processing– Data preparation



Personal database• Local storage and processing• Advantages

– Personal computers are cheap– Greater control– Friendlier interface

• Disadvantages– Replication of applications and data– Difficult to share data– Security and integrity are lower– Disposable systems– Misdirection of attention and resources



Host/terminal

• Remote storage and processing

• Associated with mainframe computers

• All shared resources are managed by the host

• Upgrades are in large chunks



Host/terminal

DCmanager

Application#1

Application#2

DBMS

Operating system

Terminal #1

Terminal #2

Terminal #3

Host



LAN architectures

• A LAN connects computers within a geographic area

• Transfer speeds of up to 1,000 Mbits/sec

• Permits sharing of devices

• A server is a computer that provides and controls access to a shareable resource



File/server

• A central data store for users attached to a LAN• Files are stored on a file/server• Data is processing on users’ personal computer• Entire files are transmitted on the LAN• Can result in heavy LAN traffic• File is locked when retrieved for update• Limited to small files and low demand



File/server

Application #3

DCmanager

Operating system

DBMSApplication #2

Operating system

DCmanager

Application#2

Operating system

DBMS

DCmanager

Filemanager

DCmanager

Application#1

Operating system

DBMSLAN



DBMS/server

• A server runs a DBMS• Only necessary records are transmitted on the

LAN• Less LAN traffic than file/server• Back-end program on the server handles retrieval• Front-end program on the client handles

processing and presentation• More sharing of processing than file/server



DBMS/server

Application #3

Operating system

Application #2

Operating system

Application#2

Operating system

DCmanager

DBMSApplication

#1

Operating system

LANDC

manager

DCmanager

DCmanager



Client/server

• File/server and DBMS/server are examples of client/server

• Objective is to reduce processing costs by splitting processing between clients and the server

• Client is typically a GUI microcomputer• Savings

– Ease of use / fewer errors– Less training



Client/server

• Costs lowered if – Some processing can be shifted from server to clients

– GUI gives productivity gains

• Cost increases– Shift from terminals to personal computers

– Rewriting software

• Client/server may not be viable for some large scale transaction processing systems



Client/Server - 2nd Generation

DCmanager DBMS

Operating system

DCmanager Application

Application server Data server

Operating system

DCmanager

Operating system

DCmanager

Browser

Thin client

Operating system

DCmanager

Browser

Operating system

LAN



Two-tier versus three-tier

Type of client Fat ThinTechnology LAN WebApplication logic Mostly on the client Mostly on the serverNetwork load Medium LowData storage Server ServerServer intelligence Medium High



Advantages of the three-tier model

• Security

• Performance

• Access to systems



Evolution of client/server computing

Architecture DescriptionTwo-tier Processing is split between client PC and

server, which also runs the DBMS.Three-tier Client PC does presentation, processing is

done by the server, and the DBMS is on aseparate server.

N-tier Client PC does presentation. Processingand DBMS can be spread across multipleservers. A distributed resourcesenvironment.



Distributed database

• Communication charges are a key factor in total processing cost

• Transmission costs increase with distance– Local processing saves money

• A database can be distributed to reduce communication costs



Distributed database

• Database is physically distributed as semi-independent databases

• There are communication links between each of the databases

• Appears as one database



A hybrid

• Architecture evolves– Old structures cannot be abandoned– New technologies offer new opportunities

• Ideally, the many structures are patched together to provide a seamless view of organizational databases

• Distributed database principles apply to this hybrid architecture



Fundamental principles

• Transparency

• No reliance on a central site

• Local autonomy

• Continuous operation

• Distributed query processing

• Distributed transaction processing



Fundamental principles

• Replication independence

• Fragmentation independence

• Hardware independence

• Operating system independence

• Network independence

• DBMS independence Independence



Distributed database access

• Remote Request

• Remote Transaction

• Distributed Transaction

• Distributed Request



Distributed database design

• Horizontal Fragmentation

• Vertical Fragmentation

• Hybrid Fragmentation

• Replication



Horizontal fragmentation

C1 C2 C3 C4 C6C5

C1 C2 C3 C4 C6C5

C1 C2 C3 C4 C6C5

C1 C2 C3 C4 C6C5

Server 3

Server 2

Server 1



Vertical fragmentation

databasesystemer e2002lene pries-hejedata structure, storage and processing architectures...

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