Chapter 4. INTERNAL REPRESENTATION OF FILES

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Chapter 4. INTERNAL REPRESENTATION OF FILES

• THE DESIGN OF THE UNIX OPERATING SYSTEM
• Maurice J. bach Prentice Hall

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Contents

• Inodes
• Structure of a regular file
• Directories
• Conversion of a path name to an inode
• Super block
• Inode assignment to a new file
• Allocation of disk blocks
• Other file types

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File System Algorithms
Lower Level File system Algorithms
namei alloc free
ialloc ifree iget iput bmap buffer
allocation algorithms getblk brelse bread
breada bwrite
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4.1 Inode

• contains the information necessary for a process
  to access a file
• exits in a static form on disk and the kernel
• reads them into an in-core inode

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4.1 Inodes

• consists of
• - file owner identifier
• - file type
• - file access permissions
• - file access times
• - number of links to the file
• - table of contents for the disk address of
  data in a file
• - file size

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4.1 Inodes

• in-core copy of the inode contains
• - status of the in-core inode
• - logical device number of file system
• - inode number
• - pointers to other in-core inodes
• - reference count

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Algorithm iget

• 1. The kernel finds the inode in inode cache and
  it is on inode free list
• remove from free list
• increment inode reference count
• 2. The kernel cannot find the inode in inode
  cache so it allocates a inode from inode free
  list
• remove new inode from free list
• reset inode number and file system
• remove inode from old hash queue,
  place on new one
• read inode from disk(algorithm bread)
• initialize inode

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Algorithm iget

• 3. The kernel cannot find the inode in inode
  cache but finds the free list empty
• error
• process have control over the allocation
  of inodes at
• user level via execution of open and close
  system calls
• and consequently the kernel cannot
  guarantee when an
• inode will become available
• 4. The kernel finds the inode in inode cache
  but it was locked
• sleep until inode becomes unlocked

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iget (inode_no) //getIncoreInode

• while (not done)
• if (inode in inode cache)
• if (inode locked)
• sleep(event inode becomes unlocked)
• continue
• if (inode on inode free list)
• remove from free list
• return locked inode
• if (no inode on free list) return error
• remove new inode from free list
• set inode number
• remove inode from old hash queue and place on new
  one
• read inode from disk
• set reference count 1
• return locked indoe

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Algorithm iput

• - The kernel locks the inode if it has not been
  already locked
• - The kernel decrements inode reference count
• - The kernel checks if reference count is 0 or
  not
• - If the reference count is 0 and the number of
  links to the file is 0, then the kernel releases
  disk blocks for file(algorithm free), free the
  inode(algorithm ifree)
• If the file was accessed or the inode was
  changed or the
• file was changed , then the kernel updates
  the disk inode
• The kernel puts the inode on free list
• - If the reference count is not 0, the kernel
  releases the inode lock

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iput (inode_no) //releaseIncoreInode

• lock inode if not locked
• decrement inode refernece count
• if (refernce count0)
• if (inode link0)
• free disk block
• set file type to 0
• free inode
• if (file accessed or inode changed or file
  changed)
• update disk inode
• put inode on free list
• Release inode lock

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4.2 Structure of a regular file
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4.2 Structure of a regular file

• Suppose System V UNIX
• Assume that a logical on the file system holds
  1K bytes and that a block number is addressable
  by a 32 bit integer, then a block can hold up to
  256 block numbers
• 10 direct blocks with 1K bytes each10K bytes
• 1 indirect block with 256 direct blocks
  1K256256K bytes
• 1 double indirect block with 256 indirect
  blocks
• 256K25664M bytes
• 1 triple indirect block with 256 double
  indirect blocks
• 64M25616G bytes

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4.2 Structure of a regular file

• Processes access data in a file by byte offset
  and view a file as a stream of bytes
• The kernel accesses the inode and converts the
  logical file block into the appropriate disk
  block
• algorithm bmap
• - The kernel calculates logical block number in
  file from byte offset
• - The kernel calculates start byte in block for
  I/O
• - The kernel calculates number of bytes to copy
  to user
• - The kernel checks if read-ahead is
  applicable, then marks inode
• - The kernel determines level of indirection

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4.2 Structure of a regular file

• - While its not at necessary level of
  indirection,
• the kernel calculates index into inode or
  indirect block from logical block number in file,
  gets disk block number from inode or indirect
  block and release buffer from previous disk read
• If there is no more levels of indirection
  , the kernel stops conversing
• Otherwise the kernel reads indirect disk
  block(bread) and adjusts logical block number in
  file according to level of indirection

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4.3 Directories

• A directory is a file whose data is a sequence of
  entries, each consisting of an inode number and
  the name of a file contained in the directory
• Path name is a null terminated character string
  divided by slash (/)
• Each component except the last must be the name
  of a directory, last component may be a
  non-directory file
• Directory layout for /etc
• Byte Offset in Directory Inode
  Number File Name
• 0 83 .
• 16 2
  ..
• 32 1798
  init

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4.4 Path conversion to an inode

• if (path name starts with root)
• working inode root inode
• else
• working inode current directory inode
• while (there is more path name)
• read next component from input
• read directory content
• if (component matches an entry in directory)
• get inode number for matched component
• release working inode
• working inodeinode of matched component
• else
• return no inode
• return (working inode)

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Algorithm namei

• - If path name starts from root, then the
  kernel assigns root inode(iget) to working inode
• - Otherwise, the kernel assigns current
  directory inode to working inode
• - While there is more path name, the kernel
  reads next path name component from input, and
  verifies that working inode is of directory,
  access permissions OK
• If working inode is of root and component
  is .., then the kernel checks whether there
  is more path name or not
• Otherwise the kernel reads directory by
  repeated use of bmap,bread,brelse

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Path conversion to an inode

If the kernel finds a match, it records the
inode number of the matched directory entry,
releases the block and the old working inode, and
allocates the inode of the match component If
the kernel does not match the path name in the
block, it releases the block, adjusts the byte
offset by the number of bytes in a block,
converts the new offset to a disk block number
and reads the new block
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4.5 Super block

• File System
• consists of
• - the size of the file system
• - the number of free blocks in the file system
• - a list of free blocks available on the file
  system
• - the index of the next free block in the free
  block list
• - the size of the inode list
• - the number of free inodes in the file system
• - a list of free inodes in the file system
• - the index of the next free inode in the free
  inode list
• - lock fields for the free block and free inode
  lists
• - a flag indicating that the super block has
  been modified

boot block super block inode list data
blocks
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4.6 Inode assignment to a new file

• Locked inode illoc()
• while (not done)
• If (super block locked)
• Sleep (event super block becomes free)
• Continue
• If (inode list in super block empty)
• Lock super block
• Get remember inode for free inode search
• Search until super block full or no more free
  inode
• Unlock super block and wake up (event super block
  free)\
• If no free inode found on disk return error
• Set remmbered inode for next free inode search
• Get inode number from super block inode list
• Get inode
• Write inode to disk
• Decrement free inode count
• Return inode

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4.6 Inode assignment to a new file

• algorithm ialloc assigns a disk inode to a
  newly created file
• -super block is unlocked
• 1.There are inodes in super block inode list
  and inode is free
• get inode number from super block inode list
• get inode (iget)
• initialize inode
• write inode to disk
• decrement file system free inode count
• 2. There are inodes in super block inode
  list but inode is not free
• get inode number from super block inode list
• get inode (iget)
• write inode to disk
• release inode (iput)

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4.6 Inode assignment to a new file

• 3. Inode list in super block is empty
• lock super block
• get remembered inode for free inode
  search
• search disk for free inode until super block
  full or
• no more free inodes(bread and brelse)
• unlock super block
• super block becomes free
• if no free inodes found on disk , stop
• otherwise, set remembered inode for next free
  inode
• search
• - If super block is locked, sleep

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4.6 Inode assignment to a new file
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4.6 Inode assignment to a new file
remembered inode
index
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Algorithm ifree

• - The kernel increments file system free inode
  count
• - If super block is locked, avoids race
  conditions by returning
• - If super block is unlock and inode list is
  full ,
• If inode number is less than remembered
  inode for search,
• then the kernel remembers the newly freed inode
  number,
• discarding the old remembered inode
  number from the super
• block
• - If super block is unlock and inode list is
  not full, then the kernel
• stores inode number in inode list

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Freeing inode

• ifree(inode_no)
• Increment free inode count
• If super block locked return
• If (inode list full) //at super block
• if (inode number ltremembered inode)
• Set remembered inode as input inode
• Else
• Store inode number in inode list
• return

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4.6 Inode assignment to a new file
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4.6 Inode assignment to a new file

• A Race Condition Scenario in Assigning Inodes
• Consider three processes A, B, and C and suppose
  that the kernel, acting on behalf of process A,
  assigns inode I but goes to sleep before it
  copies the disk inode into the in-core copy.
• While process A is asleep, suppose process B
  attempts to assign a new inode but free inode
  list is empty, and attempts assign free inode at
  an inode number lower than that of the inode that
  A is assigning.
• Suppose process C later requests an inode and
  happens to pick inode I from the super block free
  list

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4.6 Inode assignment to a new file
Process A Process B
Process C
Assigns inode I from super block
Sleeps while reading inode(a)
Tries to assign inode from super block Super
block empty(b) Search for free inodes on disk,
puts inode I in super block (c)
use the lock
Inode I in core Does usual activity
Completes search, assigns another inode(d)
Assigns inode I from super block I is in
use! Assign another inode(e)
Race Condition in Assigning Inodes
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4.7 Allocation of disk blocks
linked list of free disk block number
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Algorithm alloc

• - The kernel wants to allocate a block from a
  file system
• it allocates the next available block in
  the super block list
• - Once allocated , the block cannot be
  reallocated until it becomes free
• - If the allocated block is the last block ,
  the kernel treats it as a pointer to a block that
  contains a list of free blocks
• The kernel locks super block, reads
  block jut taken from
• free list, copies block numbers in
  block into super
• block, releases block buffer,and
  unlocks super block
• - Otherwise,
• The kernels gets buffer for block
  removed from super
• block list , zero buffer contents,
  decrements total count
• of free blocks, and marks super block
  modified

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4.7 Allocation of disk blocks
super block list
109
109
211 208 205 202 .. 112
original configuration
109 949 ..
109
211 208 205 202 . 112
After freeing block number 949
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4.7 Allocation of disk blocks
109 ..
109
211 208 205 202 . 112
After assigning block number(949)
211 208 205 202 112
211
344 341 338 335 . 243
After assigning block number(109) replenish
super block free list
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4.8 Other file types

• pipe
• - fifo(first-in-first-out)
• - its data is transient Once data is read from
  a pipe, it cannot be read again, no deviation
  from that order
• - use only direct block
• special file
• - include block device, character device
• - the inode contains the major and minor device
  number
• - major number indicates a device type such as
  terminal or disk
• - minor number indicates the unit number of the
  device