Allocation methods continuous method.47
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Allocation Methods
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HOME PREVIOUS TOPIC NEXTPREVIOUS QUESTION PAPERS FOR OSCPP TUTORIALS
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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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