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Allocation Methods

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Recap

In the last class, you have learnt:

• Various free space management techniques

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Objectives

On completion of this period, you would be able to know

• Why allocation methods are needed

• Various allocation methods

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• An allocation method refers to how disk blocks

are allocated for files

• Allocation methods are for

- effective disk space utilization

– allow fast file access

Allocation Methods

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Allocation Methods

Three major methods of allocation of disk space are

– Contiguous

– Linked

– Indexed

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Contiguous Allocation

• Each file occupies a set of contiguous blocks on

the disk

• Simple – only starting location (block #) and length (number of blocks) are required

• Random access

• Wasteful of space (dynamic storage-allocation problem)

• Files cannot grow

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Contiguous Allocation

• Accessing block b+1 after block b

(assuming that only one job is accessing the disk )

– Requires no head movement

– When needed only one track to the next

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Contiguous Allocation

• The number of disk seeks are Minimal

• The seek time when a seek is needed Minimal

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Contiguous Allocation

• Contiguous allocation file is defined by

– Disk address

– Length (in block units) of the block

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Contiguous Allocation

Example: • If file is n block long and starts at location b, then

– The blocks occupied are b, b+1, b+2…..b+n-1

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Contiguous Allocation

• The directory entry for file has

– Name

– Starting address

– Length

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Contiguous allocation

Example:

Fig 1

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Contiguous Allocation

Fig 2

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Problems of Contiguous Allocation

• Finding space for a new file or a resized file.

• Determining size requirements.

• External fragmentation of the hard disk.

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Wasteful if only a few pointers are stored in the index block

Number of disk access necessary to retrieve the address of target block address

Indexed Allocation Disadvantages

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In this class, you have learnt

Various allocation methods

Their advantages and disadvantages

Summary

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Frequently Asked Questions

1.Why allocation methods are needed

2. Explain various allocation methods

3.List the advantages and disadvantages of

various allocation methods

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Quiz

• Allocation methods are for effective disk space utilization [T/F]

• With contiguous allocation the disk movement is minimal [T/F]

True

True

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Quiz

Linked allocation support direct access [T/F]– False

Space is wasted for pointers [T/F]– True

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