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Allocation methods continuous method.47

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Allocation methods continuous method.47

  1. 1. Allocation Methods http://raj-os.blogspot.in/ 1
  2. 2. HOME PREVIOUS TOPIC NEXTPREVIOUS QUESTION PAPERS FOR OSCPP TUTORIALS http://raj-os.blogspot.in/ 2
  3. 3. RecapIn the last class, you have learnt:• Various free space management techniques http://raj-os.blogspot.in/ 3
  4. 4. ObjectivesOn completion of this period, you would be able to know• Why allocation methods are needed• Various allocation methods http://raj-os.blogspot.in/ 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 http://raj-os.blogspot.in/ 5
  6. 6. Allocation MethodsThree major methods of allocation of disk space are – Contiguous – Linked – Indexed http://raj-os.blogspot.in/ 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 http://raj-os.blogspot.in/ 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 http://raj-os.blogspot.in/ 8
  9. 9. Contiguous Allocation• The number of disk seeks are Minimal• The seek time when a seek is needed Minimal http://raj-os.blogspot.in/ 9
  10. 10. Contiguous Allocation• Contiguous allocation file is defined by – Disk address – Length (in block units) of the block http://raj-os.blogspot.in/ 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 http://raj-os.blogspot.in/ 11
  12. 12. Contiguous Allocation• The directory entry for file has – Name – Starting address – Length http://raj-os.blogspot.in/ 12
  13. 13. Example: Fig 1 Contiguous allocation http://raj-os.blogspot.in/ 13
  14. 14. Contiguous Allocation Fig 2 http://raj-os.blogspot.in/ 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. http://raj-os.blogspot.in/ 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 http://raj-os.blogspot.in/ 16
  17. 17. SummaryIn this class, you have learnt Various allocation methods Their advantages and disadvantages http://raj-os.blogspot.in/ 17
  18. 18. Frequently Asked Questions1.Why allocation methods are needed2. Explain various allocation methods3.List the advantages and disadvantages of various allocation methods http://raj-os.blogspot.in/ 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 http://raj-os.blogspot.in/ 19
  20. 20. QuizLinked allocation support direct access [T/F] – FalseSpace is wasted for pointers [T/F] – True http://raj-os.blogspot.in/ 20
  21. 21. Other subject materials• Web designing• Micro processors• C++ tutorials• javahome http://raj-os.blogspot.in/ 21

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