PPT

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