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Free Space Management

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Recap

In the last class, you have learnt:

• Secondary memory, types of secondary memory

• Various types of heads, advantages of one over other

• Techniques for increasing storage capacity

• Characteristics of the disk

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Objectives

On completion of this period, you would be able to

know

• Need for free space management

• Various implementations of free space lists

• Bit vector, Linked list, Grouping, Counting

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Understand Free Space Management

• Need for free space management

– Limited amount of disk space

– Necessary to reuse the space from deleted files

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Need for Free Space Management

• To keep track of disk space :

– The system maintains a free space list

– The free space list records all disk blocks that are free

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File Creation & Deletion

• File creation

– Search the free space list of the required amount of space

– Allocate the space to the new file

– Remove the space from free space list

• File deletion

– Add the disk space to the free space list

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Various Implementations of Free Space List

• Bit Vector

• Linked List

• Grouping

• Counting

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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

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Bit Vector

• Example:

– Consider a disk, where blocks

2,3,4,5,8,9,10,11,12,13,17,18,25,26 & 27 are free

and the rest of the blocks are allocated

– The free-space bit map would be

– 001111001111110001100000011100000……

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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

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• Bit map requires extra space

Example:

– Disk Size = 1.3 GB

– Block size = 512 bytes

– Need a bit map of over 332 KB to track its free

blocks

Bit Vector

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Bit Vector

• Advantage:

– Relatively Simple and Efficient

– Easy to get contiguous files

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Linked List

• Approach

– 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

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Linked Free Space List on Disk

Example:

• As shown in figure in the next slide, there is a

• 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

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Linked free space list on disk Fig 1

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Linked List

• Linked list (free list)– Cannot get contiguous space easily

– No waste of space

– Not efficient for faster access

– Substantial I/O

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Grouping

• Approach

– 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, and so on as shown in the next slide

Fig : 2

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Grouping

Block No

.

Block No

.

.

Block No

Block No

.

Block No

Block No

Block No

.

.

.

Block No

Block No

. . .

First Free block Fig 2

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Grouping

• Advantage

– Large number of free blocks can be found quickly

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Counting

• Approach

– 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 count21

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Counting

• Entry in a free space list consists of

– Disk address

– Count

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Counting

Constraints:

• Each entry requires more space than a simple

disk address

• The overall list will be shorter, as long as the

count is greater than one

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Summary

In this class, you have learnt:

• What is free space management

• Various free space management techniques

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Frequently asked questions

1. Explain the need for free space management

2. List the various implementations of free space list

3. Explain bit vector

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Frequently Asked Questions

4. Explain the following free space management techniques

a) linked list

b) Grouping

c) Counting

5. Write the advantages and disadvantages of linked list method

of free space management

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Quiz

• To keep track of disk space the system maintains

• Bit map requires extra space [T/F]

Free space list

True

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Quiz

• In Linked allocation it is easy to get the contiguous space [T/F]

• In Counting it is easy to get the contiguous space [T/F]

False

True

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