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