UNIX File Systems (Chap 4. in the book

1
UNIX File Systems(Chap 4. in the book the
design of the UNIX OS)
Acknowledgement Soongsil Univ. Presentation
Materials
2
File System

• Abstraction used by the kernel to represent and
  organize the systems storage resources
• Properties
• Hierarchical structure
• Ability to create and delete files
• Dynamic growth of files
• Protection of file data
• Treatment of peripheral devices as files

3
UNIX File System Overview
4
UNIX File System Overview
5
File System Layout
6
Sample File System
7
Table of 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
• Summary

8
Summary

• Inode is the data structure that describes the
  attributes of a file, including the layout of its
  data on disk.
• Two version of the inode
• Disk copy store the inode information when file
  is not in use
• In-core copy record the information about
  active files.
• ialloc/ifree assignment of a disk inode
• iget/iput allocation of in-core inodes
• bmap locate disk blocks of a file, according to
  byte offset
• Directories files that correlate file name
  components to inode numbers
• namei convert file names to inodes
• alloc/free assignment of new disk blocks to a
  file

9
Table of 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
• Summary

10
Definition of Inodes

• Every file has a unique inode
• Contain the information necessary for a process
  to access a file
• Exist in a static form on disk
• Kernel reads them into an in-core inode to
  manipulate them.

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Contents of Disk Inodes

• File owner identifier (individual/group owner)
• File type (regular, directory,..)
• File access permission (owner,group,other)
• File access time
• Number of links to the file (chap5)
• Table of contents for the disk address of data in
  a file (byte stream vs discontiguous disk blocks)
• File size
• Inode does not specify the path name that
  access the file

12
Sample Disk Inode

• File owner identifier
• File type
• File access permission
• File access time
• Number of links to the file
• Table of contents for the disk address of data in
  a file
• File size

• Owner mjb
• Group os
• Type regular file
• Perms rwxr-xr-x
• Accessed Oct 23 1984 145 P.M
• Modified Oct 22 1984 103 A.M
• Inode Oct 23 1984 130 P.M
• Size 6030 bytes
• Disk addresses

13
Distinction Between Writing Inode and File

• File change only when writing it.
• Inode change when changing the file, or when
  changing its owner, permisson,or link settings.
• Changing a file implies a change to the inode,
• But, changing the inode does not imply that the
  file change.

14
Contents of The In-core copy of The Inode

• Fields of the disk inode
• Status of the in-core inode, indicating whether
• Inode is locked
• Process is waiting for the inode to become
  unlocked
• Differ from the disk copy as a result of a change
  to the data in the inode
• Differ from the disk copy as a result of a change
  to the file data
• File is a mount point

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Contents of The In-core copy of The Inode

• Logical device number of the file system
• Inode number (linear array on disk, disk inode
  not need this field)
• Pointers to other in-core inodes
• Reference count

16
Accessing Inodes

• Kernel identifies inodes by their file system and
  inode number
• Allocate in-core inodes at the request of
  higher-level algorithms (in-core inode, by iget
  algorithm)
• Kernel maps the device number inode number into
  a hash queue
• Search the queue for the inode

17
Block Number Byte Offset

• Computing logical disk block number
• Block number
• ((inode number 1) / number of inodes per
  block)
• start block inode list
• Computing byte offset of the inode in the block
• ((inode number 1) mod (number of inodes per
  block))
• size of disk inode

18
Inode Lock and Reference Count

• Kernel manipulates them independently
• Inode lock
• Set during execution of a system call to prevent
  other processes from accessing the inode while it
  is in use.
• Kernel releases the lock at the conclusion of the
  system call
• Inode is never locked across system calls.
• Reference count
• Kernel increase/decrease when reference is
  active/inactive
• Prevent the kernel from reallocating an active
  in-core inode

19
Table of 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
• Summary

20
Direct and Indirect Blocks in Inode
21
Byte Capacity of a File

• System V UNIX. Assume that
• Run with 13 entries
• 1 logical block 1K bytes
• Block number address a 32 bit (4byte) integer
• 1 block can hold up to 256 block number (1024byte
  / 4byte)
• 10 direct blocks with 1K bytes each10K bytes
• 1 indirect block with 256 direct blocks
  1K256256K bytes
• 1 double indirect block with 256 indirect
  blocks256K25664M bytes
• 1 triple indirect block with 256 double indirect
  blocks64M25616G
• Size of a file 4G (232), if file size field in
  inode is 32bits

22
Byte Offset and Block Number

• Process access data in a file by byte offset.
• The file starts at logical block 0 and continues
  to a logical block number corresponding to the
  file size
• Kernel accesses the inode and converts the
  logical file block into the appropriate disk
  block (bmap algorithm)

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Conversion of Byte Offset to Block Number

• Algorithm bmap / block map of logical file byte
  offset to file system block /
• Input inode, byte offset
• Output (1)block number in file system, (2)byte
  offset into block,
• (3)bytes of I/O in block, (4)read
  ahead block number
• calculate logical block number in file from byte
  offset
• calculate start byte in block for I/O /
  output 2 /
• calculate number of bytes to copy to user /
  output 3 /
• check if read-ahead applicable, mark inode /
  output 4/
• determine level of indirection
• while(not at necessary level of indirection)
• calculate index into inode or indirect block
  from logical block number in file
• get disk block number from inode or indirect
  block
• release buffer from previous disk read, if any
  (algorithm brelse)
• if(no more levels of indirection) return (block
  number)
• read indirect disk block (algorithm bread)
• adjust logical block number in file according
  to level of indirection

24
Block Layout of a Sample File and Its inode
0
Byte 9000 in a file -gt 8block 808th byte
8
8
816th byte
(10K256K)
Byte 350,000 in a file
11
25
Block Entry in the Inode is 0

• Logical block entry contain no data.
• Process never wrote data into the file at that
  byte offset
• No disk space is wasted
• Cause by using the lseek and write system call

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Two Extensions to the inode Structure

• 4.2 BSD file system
• The more data the kernel can access on the disk
  in a single operation, the faster file access
  becomes
• But it increase block fragmentation
• Solution one disk block can contain fragments
  belonging to several files
• To store file data in the inode
• By expanding the inode to occupy an entire disk
  block
• The remainder can store the entire file

27
Table of 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
• Summary

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Directories

• A directory is a file
• Its 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 /
• Each component except the last must be the name
  of a directory, last component may be a
  non-directory file

29
Directory Layout for /etc
Byte Offset in Directory Inode Number (2 bytes) File Names
0 16 32 48 ... 224 240 256 83 2 1798 1276 ... 0 95 188 . .. init fsck crash mkfs inittab
30
Table of 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
• Summary

31
Algorithm for Conversion of a Path Name to an
Inode

• Algorithm namei / convert path name to inode /
• Input path name
• Output locked inode
• if(path name starts from root) working inode
  root inode (algorithm iget)
• else working inode current directory inode
  (algorithm iget)
• while(there is more path name)
• read next path name component from input
• verify that working inode is of
  directory,access permission OK
• if(working inode is of root and component is
  ..)
• continue / loop back to while /
• read directory (working inode) by repeated use
  of algorithms
• bmap,bread and brelse

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Algorithm for Conversion of a Path Name to an
Inode

• if(component matches an entry in directory
  (working inode))
• get inode number for matched component
• release working inode (algorithm iput)
• working inodeinode of matched
  component(algorithm iget) else / component
  not in directory
• return (no inode)
• return (working inode)

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Table of 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
• Summary

34
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
35
Table of 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
• Summary

36
Inode Assignment to a New File

• File system contains a linear list of inodes
• Inode is free its type field is zero (0)
• Super block contains an array to cache the
  numbers of free inodes in the file system (to
  improve performance)

37
Algorithm for Assigning New Inodes

• Algorithm ialloc / allocate inode /
• Input file system
• Output locked inode
• while(not done)
• if(super block locked)
• sleep(event super block becomes free)
  continue
• if(inode list in super block is empty)
• lock super block
• get remembered inode for free inode search
• search disk for free inodes until super block
  full,
• or no more free inodes (bread and brelese)
• unlock super block
• wake up (event super block becomes free)
• if(no free inodes found on disk) return (no
  inode)
• set remembered inode for next free inode
  search

38
Algorithm for Assigning New Inodes

• / there are inodes in super block inode list
  /
• get inode number from super block inode list
• get inode (algorithm iget)
• if(inode not free after all)
• write inode to disk
• release inode (algorithm iput)
• continue / while loop /
• / inode is free /
• initialize inode
• write inode to disk
• decrement file system free inode count
• return (inode)
• // end of while

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Assigning Free Inode from Middle of List
40
Assigning Free Inode Super Block List Empty
41
Algorithm for Freeing Inode

• Algorithm ifree / inode free /
• Input file system inode number
• Output none
• increment file system free inode count
• if(super block locked) return
• if(inode list full)
• if(inode number less than remembered inode for
  search)
• set remembered inode for search input inode
  number
• else
• store inode number in inode list
• return

42
Placing Free Inode Numbers Into the Super Block
43
Table of 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
• Summary

44
Linked List of Free Disk Block Numbers
45
Algorithm for Allocating Disk Block

• Algorithm alloc / file system block allocation
  /
• Input file system number
• Output buffer for new block
• while(super block locked) sleep (event super
  block not locked)
• remove block from super block free list
• if(removed last block from free list)
• lock super block
• read block just taken from free list (algorithm
  bread)
• copy block numbers in block into super block
• release block buffer (algorithm brelse)
• unlock super block
• wake up processes (event super block not
  locked)

46
Algorithm for Allocating Disk Block

• get buffer form block removed from super block
  list (algorithm getblk)
• zero buffer contents
• decrement total count of free blocks
• mark super block modified
• return buffer

47
Requesting and Freeing 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
48
Requesting and Freeing 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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Table of 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
• Summary

50
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
• Use only direct block (not the indirect block)
• Special file
• 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