Chapter%204%20:%20File%20Systems

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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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Files (Cont.)

• Structuring
• (a) Byte sequence (as in DOS, Windows UNIX)
• (b) Record sequence (as in old systems)
• (c) Tree structure (as in some mainframe Oses)

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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

1. Append
2. Seek
3. Get attributes
4. Set Attributes
5. 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)
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
Sector
Track

• 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

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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)
FAT (File allocation table)
Disk size
EOF
Free
5
Free
File blocks
7
Bad
First block address is in directory entry
1

n
Free
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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 dont take
  up space in actual block
• Random access is faster since FAT is always in
  memory
• 16-bit DOS FAT length is (655362)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?

Clustering
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i (index)-nodes (UNIX)
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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 lt 10 KB
• Single indirect file size lt 25610 266 KB
• Double indirect file size lt 256256 266
  65802 KB 64.26 MB
• Triple indirect file size lt 256256256
  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)
Attributes (A,D,V,S,H,R)
Pointer to first data block
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UNIX Directory Structure (16 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
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

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Disk Space Management
Block size

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

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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

Process tables
i-node of file