Ceng 334 - Operating Systems 4-1

Chapter 4 : File Systems

• What is a file system?

• Objectives & user requirements

• Characteristics of files & directories

• File system implementation

• Directory implementation

• Free blocks management

• Increasing file system performance

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File System

• The collection of algorithms and data structures which perform the translation from logical file operations (system calls) to actual physical storage of information

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Objectives of a File System

• Provide storage of data and manipulation

• Guarantee consistency of data and minimise errors

• Optimise performance (system and user)

• Eliminate data loss (data destruction)

• Support variety of I/O devices

• Provide a standard user interface

• Support multiple users

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User Requirements

• Access files using a symbolic name

• Capability to create, delete and change files

• Controlled access to system and other users’ files

• Control own access rights

• Capability of restructuring files

• Capability to move data between files

• Backup and recovery of files

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Files• Naming

– Name formation

– Extensions (Some typical extensions are shown below)

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• Structuring

– (a) Byte sequence (as in DOS, Windows & UNIX)

– (b) Record sequence (as in old systems)

– (c) Tree structure (as in some mainframe Oses)

Files (Cont.)

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Files (Cont.)

• File types– Regular (ASCII, binary)– Directories– Character special files– Block special files

• File access– Sequential access– Random access

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Files (Cont.)• File attributes

– Read, write, execute, archive, hidden, system etc.– Creation, last access, last modification

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Files (Cont.)• File operations

1. Create

2. Delete

3. Open

4. Close

5. Read

6. Write

7. Append

8. Seek

9. Get attributes

10.Set Attributes

11.Rename

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Directories• Where to store attributes

– In directory entry (DOS, Windows)– In a separate data structure (UNIX)

• Path names– Absolute path name– Relative path name– Working (current) directory

• Operations– Create, delete, rename, open directory, close

directory, read directory, link (mount), unlink

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Directories & Files (UNIX)

d2

f3f2

f1

/

d1 f4

f5 f7

d3

d4 d5 d6

f6

Disk ADisk B

Root Directory

Linked Branch

Working Directory

• Working directory : d2 • Absolute path to file f2 : /d1/d2/f2• Relative path to file f2 : f2

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Physical Disk Space Management

Heads

Cylinder

• Each plate is composed of sectors or physical blocks which are laid along concentric tracks

• Sectors are at least 512 bytes in size• Sectors under the head and accessed without a

head movement form a cylinder

SectorTrack

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File System Implementation

• Contiguous allocation

• Linked list allocation

• Linked list allocation using an index (DOS file allocation table - FAT)

• i-nodes (UNIX)

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Contiguous Allocation• The file is stored as a contiguous block of data

allocated at file creation

(a) Contiguous allocation of disk space for 7 files

(b) State of the disk after files D and E have been removed

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Contiguous Allocation (Cont.)

• FAT (file allocation table) contains file name, start block, length

• Advantages– Simple to implement (start block & length is

enough to define a file)– Fast access as blocks follow each other

• Disadvantages– Fragmentation– Re-allocation (compaction)

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Linked List Allocation• The file is stored as a linked list of blocks

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Linked List Allocation (Cont.)

• Each block contains a pointer to the next block• FAT (file allocation table) contains file name, first

block address• Advantages

– Fragmentation is eliminated– Block size is not a power of 2 because of

pointer space• Disadvantages

– Random access is very slow as links have to be followed

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Linked list allocation using an index (DOS FAT)

Disk size

EOF

1

Free

5

Free

7

Bad

Free

…..

3 75 1

0

1

2

3

4

5

6

7

FAT (File allocation table)

File blocks

n

First block address is in directory entry

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Linked list allocation using an index (Cont.)

• The DOS (Windows) FAT is arranged this way

• All block pointers are in FAT so that don’t take up space in actual block

• Random access is faster since FAT is always in memory

• 16-bit DOS FAT length is (65536+2)*2 = 131076 bytes

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Problem

• 16-bit DOS FAT can only accommodate 65536 pointers (ie., a maximum of 64 MB disk)

• How can we handle large disks such as a 4 GB disk?

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i (index)-nodes (UNIX)File mode

Number of linksUIDGID

File sizeTime created

Time last accessedTime last modified

10 disk block numbersSingle indirect block

Triple indirect blockDouble indirect block

Indirect blocks Data blocks

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i-nodes (Cont.)

• Assume each block is 1 KB in size and 32 bits (4 bytes) are used as block numbers

• Each indirect block holds 256 block numbers• First 10 blocks : file size <= 10 KB• Single indirect : file size <= 256+10 = 266 KB• Double indirect : file size <= 256*256 +266 =

65802 KB = 64.26 MB• Triple indirect : file size <= 256*256*256 +

65802= 16843018 KB = ~16 GB

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Directory Implementation

• DOS (Windows) directory structure

• UNIX directory structure

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DOS (Windows) Directory Structure (32 bytes)

File name Ext A Reserved T PD Size

8 bytes 3 1 10 2 2 2 4

Attributes (A,D,V,S,H,R)

Time of creation

Date of creation

Pointer to first data block

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UNIX Directory Structure (16 bytes)

I-node # File name2 bytes 14 bytes

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The Windows 98 Directory Structure

•Extended MS DOS Directory Entry

•An entry for (part of) a long file name

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The Windows 98 Directory Structure

An example of how a long name is stored in Windows 98

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Path Name Lookup : /usr/ast/mbox

245

Root (/) i-node

1 .1 ..4 bin7 dev

14 lib9 etc6 usr

Root directory file block 245

132

i-node 6 of /usr

6 .1 ..

19 prog30 stu51 html26 ast45 genc

/usr directory file block 132

406

i-node 26 of /usr/ast26 .6 ..

60 mbox92 books81 src

/usr/ast directory file block 406

i-node 60 of /usr/ast/mbox

Blocks of file

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Two ways of handling long file names in a Directory

In-line In a heap

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Shared Files

• File f2 is shared by two paths (users!) and there is one physical copy.

• The directories d1 & d2 point to the same i-node with link count equal to 2

• Deletion is done by decrementing the link count. When it reaches zero the file is deleted physically

/

d1 d2

f1 f2 f3

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Disk Space Management

• Dark line (left hand scale) gives data rate of a disk• Dotted line (right hand scale) gives disk space efficiency• All files 2KB

Block size

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How to Keep Track of Free Disk Blocks

• Linked list of disk blocks

• Bit maps

• Indexing as used in DOS FAT

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Linked List of Disk Blocks

•Allocation is simple. •Delete block number from free blocks list

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Bit Maps• The bit map is implemented by

reserving a bit string whose length equals the number of blocks

• A ‘1’ may indicate that the block is used and ‘0’ for free blocks

• If the disk is nearly full then the bit map method may not be as fast as the linked list method

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Increasing File System Performance• Disks (floopies, hard disks, CD ROMS) are

still slow when compared to the memory• Use of a memory cache may speed the disk

transfers between disk and process• Blocks are read into the cache first.

Subsequent accesses are through the cache• Blocks are swapped in & out using

replacement algorithms such as FIFO, LRU• System crashes may cause data loss if

modified blocks are not written back to disk

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Where to Put the Current “File Position” Field

• The file position field is a 16 or 32 bit variable which holds the address of the next byte to be read or written in a file

– Put it in the i-node

– Put it in process table

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File Position Field in i-node

• If two or more processes share the same file, then they must have a different file position

• Since i-node is unique for a file, the file position can not be put in the i-node

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File Position Field in Process Table

• When a process forks, both the parent and the child must have the same file position

• Since the parent and the child have different process tables they can not share the same file position

• So, we can not put in process table

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Solution

• Use an intermediate table for file positions

parent

child

Process tables

position

File positions table

i-nodeof

file