1.
Free Space Management, Efficiency &
Performance, Recovery and NFS
Department of Computer Science & Engineering
Hamdard University Bangladesh

2.
Free Space Management
To keep track of disk space :
o The system maintains a free space list
o The free space list records all disk blocks that are free

3.
Various Implementations of Free Space List
 Bit Vector
 Linked List
 Grouping
 Counting

4.
Bit Vector
• Free space list is implemented as a bit map or bit vector
• If the block is free the bit is set to 1
• If the block is allocated the bit is set to 0

5.
Bit vector (n blocks)
…
0 1 2 n-1
bit[i] =
0  block[i] free
1  block[i] occupied
Block number calculation
(number of bits per word) *
(number of 0-value words) +
offset of first 1 bit
Bit Vector

6.
Bit map requires extra space. Example:
block size = 212 bytes (4 K)
disk size = 230 bytes (1 gigabyte)
n = 230/212 = 218 bits (or 32K bytes)
Easy to get contiguous files
Linked list (free list)
Cannot get contiguous space easily
No waste of space
Grouping
Large free blocks can be quickly found
Counting
Bit Vector

7.
Linked List
 Link all free disk blocks together
 Keep a pointer to the first free block
 This block contains a pointer to next free block, and so on

8.
Linked Free Space List on Disk
Example:
Pointer to block 2, as the first free block
Block 2 would contain a pointer to block 3,
Which would point to block 4,
Which would point 5,8,9,10,11,12,13,17,18,25,26,27

9.
Grouping
 Store addresses of n free blocks in the first free block
 The first n-1 of these are actually free
 The last block contains the address as of another free blocks.
Block No
.
Block No
.
.
Block No
Block No
.
Block No
Block No
Block No
.
.
.
Block No
Block No
. . .
First Free
block

10.
Counting
 Based on the fact that several contiguous blocks may be allocated and freed
simultaneously
 Holds the address of the first free block and number “n” of free contiguous blocks
that follow the first block
 Each entry is the free space list consist of a disk address and a count

11.
Counting
• Entry in a free space list consists of
– Disk address
– Count

12.
Efficiency and Performance
 Efficiency dependent on:
 disk allocation and directory algorithms
 types of data kept in file’s directory entry
 Performance
 disk cache – separate section of main memory for frequently
used blocks
 free-behind and read-ahead – techniques to optimize
sequential access
 improve PC performance by dedicating section of memory
as virtual disk, or RAM disk.

13.
Recovery Techniques
• Consistency checking – compares data in directory structure with
data blocks on disk, and tries to fix inconsistencies
• Use system programs to back up data from disk to another storage
device (floppy disk, magnetic tape, other magnetic disk, optical)
• Recover lost file or disk by restoring data from backup

14.
 An implementation and a specification of a software system for accessing
remote files across LANs (or WANs).
 The implementation is part of the Solaris and SunOS operating systems
running on Sun workstations using an unreliable datagram protocol
(UDP/IP protocol and Ethernet.
Network File System (NFS)