1. Chapter 4. INTERNAL REPRESENTATION OF FILES
THE DESIGN OF THE UNIX OPERATING SYSTEM
Maurice J. bach Prentice Hall

2. 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

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

4. 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

5. 4.1 Inodes - file owner identifier consists of - 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. iput (inode_no) //releaseIncoreInode
lock inode if not locked
decrement inode refernece count
if (refernce count==0)
if (inode link==0)
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

12. 4.2 Structure of a regular file
direct0
direct1
direct2
direct3
direct4
direct5
direct6
direct7
direct8
direct9
single indirect
double indirect
triple indirect
Inode
Data Blocks

13. 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 each=10K bytes
1 indirect block with 256 direct blocks= 1K*256=256K bytes
1 double indirect block with 256 indirect blocks=
256K*256=64M bytes
1 triple indirect block with 256 double indirect blocks=
64M*256=16G bytes

14. 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

15. 4.2 Structure of a regular file
- While it’s 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

16. 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
init

17. 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 inode=inode of matched component
return no inode
return (working inode)

18. 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

19. 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

20. 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

21. 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

22. 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
release inode (iput)

23. 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

24. 4.6 Inode assignment to a new file
free inodes empty
array1
Super Block Free Inode List
index
free inodes empty
array2
Assigning Free Inode from Middle of List

25. 4.6 Inode assignment to a new file
Assigning Free Inode – Super Block List Empty
empty
array1
Super Block Free Inode List
index
free inodes
array2
remembered inode
index

26. 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

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

28. 4.6 Inode assignment to a new file
free inodes
remembered inode
Original Super Block List of Free Inodes
index
Free Inode 499
free inodes
remembered inode
index
Free Inode 601
free inodes
remembered inode
index

29. 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

30. 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

31. 4.7 Allocation of disk blocks
…………………………..
…………………
109
………………… 214
211
…………………
310
linked list of free disk block number

32. 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

33. 4.7 Allocation of disk blocks
super block list
109 …………………………………………………………
109
…………………………….. 112
original configuration
…………………………………………………..
109
………………………………. 112
After freeing block number 949

34. 4.7 Allocation of disk blocks
109 ………………………………………………………..
109
………………………………. 112
After assigning block number(949)
……………………………… 112
211
………………………………. 243
After assigning block number(109)
replenish super block free list

35. 4.8 Other file types pipe special file - 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