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Allocation methods continuous method.47


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Allocation methods continuous method.47

  1. 1. Allocation Methods 1
  3. 3. RecapIn the last class, you have learnt:• Various free space management techniques 3
  4. 4. ObjectivesOn completion of this period, you would be able to know• Why allocation methods are needed• Various allocation methods 4
  5. 5. Allocation Methods• An allocation method refers to how disk blocks are allocated for files• Allocation methods are for - effective disk space utilization – allow fast file access 5
  6. 6. Allocation MethodsThree major methods of allocation of disk space are – Contiguous – Linked – Indexed 6
  7. 7. 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 7
  8. 8. 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 8
  9. 9. Contiguous Allocation• The number of disk seeks are Minimal• The seek time when a seek is needed Minimal 9
  10. 10. Contiguous Allocation• Contiguous allocation file is defined by – Disk address – Length (in block units) of the block 10
  11. 11. Contiguous AllocationExample:• If file is n block long and starts at location b, then – The blocks occupied are b, b+1, b+2…..b+n-1 11
  12. 12. Contiguous Allocation• The directory entry for file has – Name – Starting address – Length 12
  13. 13. Example: Fig 1 Contiguous allocation 13
  14. 14. Contiguous Allocation Fig 2 14
  15. 15. Problems of Contiguous Allocation• Finding space for a new file or a resized file.• Determining size requirements.• External fragmentation of the hard disk. 15
  16. 16. Indexed Allocation Disadvantages Wasteful if only a few pointers are stored in theindex blockNumber of disk access necessary to retrieve theaddress of target block address 16
  17. 17. SummaryIn this class, you have learnt Various allocation methods Their advantages and disadvantages 17
  18. 18. Frequently Asked Questions1.Why allocation methods are needed2. Explain various allocation methods3.List the advantages and disadvantages of various allocation methods 18
  19. 19. Quiz• Allocation methods are for effective disk space utilization [T/F] True• With contiguous allocation the disk movement is minimal [T/F] True 19
  20. 20. QuizLinked allocation support direct access [T/F] – FalseSpace is wasted for pointers [T/F] – True 20
  21. 21. Other subject materials• Web designing• Micro processors• C++ tutorials• javahome 21