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
Conversion of a path name to an inode
Super block
Inode assignment to a new file
Delete Inode

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

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. 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

8. 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

9. 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

10. 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

11. 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)

12. 4.4 Conversion of a path name to an inode
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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 4.7 Allocation of disk blocks
…………………………..
……………………
109
…………………… 214
211
……………………
310
linked list of free disk block number

20. 4.7 Allocation of disk blocks
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

21. 4.7 Allocation of disk blocks
super block list
109 …………………………………………………………
109
…………………………….. 112
original configuration
…………………………………………………..
109
………………………………. 112
After freeing block number 949

22. 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