This document discusses different file allocation methods for managing disk space, including contiguous, linked, indexed, and inode allocation. It also covers free space management techniques like bit vectors and linked lists. Inode allocation combines direct indexing, single and double indirect indexing to support very large files over 4GB by organizing file metadata and block pointers in inodes.

1. Contiguous
Allocation
1

2. Contiguous Allocation of Disk Space

3. Linked Allocation

4. File-Allocation Table (DOS, others)
•Section of disk at
start of partition
•Table with one entry
per disk block
•Indexed by block#
•Each entry contains
link to next block
•Special code for
EOF
•“0” means empty
block

5. Indexed Allocation
 Brings all pointers (for one file) together into an
index block.
 Supports both sequential and random access
• (Index into table, pointer to block)
 Wasteful for small files (use entire block for index)
• But no external fragmentation
 Logical view.
index table

6. Example of Indexed Allocation

7. Indexed Allocation – Multilevel Index
(Supports very large file size)

outer-index
index table file

8. Combined Scheme: UNIX inode
(Example with 4K bytes per block)
1212
linklink
Size:blocks, bytes
One inode table
per partition
64 bytes per inode
Directory entries
point to inode
Link count
Direct index
supports 48k file
Double indirect
supports huge
files (>4 GB)

9. Maximum File Size with inodes
 Assume 12 direct blocks, 4K blocks, 4 byte block
pointer
 Estimate maximum addressable up to the double
indirect blocks:
• Direct blocks (12) 48k
• Single indirect (4096/4=1024 ptrs to blocks) 4096k
• Double indirect (1024 of these) 4194304k
• Total 4198448k
• (> 4 GB)
 Didn’t even use triple indirect!

10. File Access With Unix inodes
 “Superblock” and
other structures
not shown
 “root” directory is
inode #2
 Directories contain
pointers to inodes
 Access path
shown to
/usr/bin/wc
 Notice 8 disk
accesses to get
first block of wc!
inodes
(root)
root
directory
usr
directory
bin
directory
wc
(file)
Data blocks
usr
bin
wc
#2
0
n

11. In-Memory File System Structures Re-Visited:
(Unix Case)
Opening
A file
Reading
A file
inode
Copy of
inode
Pointer to
Memory copy
Of inode

12. Free-Space Management
 Bit vector (n blocks)
…
0 1 2 n-1
bit[i] =

1 ⇒ block[i] free
0 ⇒ block[i] occupied
To find a free block: find the first non-zero
word, locate first 1-bit.
Block number (of first free block) is:
umber of bits per word) * (number of 0-value words) + offset of first 1

13. Bit Vector (Cont.)
 Bit map requires extra space. Example:
block size = 212
bytes
disk size = 230
bytes (1 gigabyte)
n = 230
/212
= 218
bits (or 32K bytes)
 Easy to get contiguous files
• Just look for consecutive 1’s in bit map

14. Linked List of Free Space on Disk

15. Free List Approach
 Linked list
• Hard to find contiguous space easily
• But no waste of space
 Grouping
• Store addresses of n free blocks in the first block
• Last of these addresses is to a block that contains
addresses of another n free blocks
• So many free blocks can be found at one time
 Counting
• Clusters of contiguous free blocks recorded together
• Keep list of first block address, count of contiguous
free ones