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Free Space Management                        http://raj-os.blogspot.in/   1
HOME     PREVIOUS TOPIC NEXTPREVIOUS QUESTION PAPERS FOR OSCPP TUTORIALS        http://raj-os.blogspot.in/   2
RecapIn the last class, you have learnt:• Secondary memory, types of secondary memory• Various types of heads, advantages ...
ObjectivesOn completion of this period, you would be able to know• Need for free space management• Various implementations...
Understand Free Space Management• Need for free space management  – Limited amount of disk space  – Necessary to reuse the...
Need for Free Space Management• To keep track of disk space :   – The system maintains a free space list   – The free spac...
File Creation & Deletion• File creation   – Search the free space list of the required amount of     space   – Allocate th...
Various Implementations of Free Space List• Bit Vector• Linked List• Grouping• Counting                 http://raj-os.blog...
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 b...
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...
Bit Vector (n blocks)           0 1 2                                 n-1                                         …       ...
Bit Vector• Bit map requires extra spaceExample:   – Disk Size = 1.3 GB   – Block size = 512 bytes   – Need a bit map of o...
Bit Vector• Advantage:  – Relatively Simple and Efficient  – Easy to get contiguous files                    http://raj-os...
Linked List• Approach  – Link all free disk blocks together  – Keep a pointer to the first free block  – This block contai...
Linked Free Space List on DiskExample:• As shown in figure in the next slide, there is a• Pointer to block 2, as the first...
Fig 1Linked free space list on disk         http://raj-os.blogspot.in/   16
Linked List• Linked list (free list)   – Cannot get contiguous space easily   – No waste of space   – Not efficient for fa...
Grouping• Approach  – Store addresses of n free blocks in the first free    block  – The first n-1 of these are actually f...
Block No     Block No                             Block NoBlock No     Block No                             Block No    . ...
Grouping• Advantage  – Large number of free blocks can be found quickly                   http://raj-os.blogspot.in/      20
Counting• Approach  – Based on the fact that several contiguous blocks may    be allocated and freed simultaneously  – Hol...
Counting• Entry in a free space list consists of   – Disk address   – Count                    http://raj-os.blogspot.in/ ...
CountingConstraints:   • Each entry requires more space than a simple    disk address   • The overall list will be shorter...
SummaryIn this class, you have learnt:• What is free space management• Various free space management techniques           ...
Frequently asked questions1. Explain the need for free space management2. List the various implementations of free space l...
Frequently Asked Questions4. Explain the following free space management techniquesa)   linked listb)   Groupingc)   Count...
Quiz• To keep track of disk space the system maintains       Free space list• Bit map requires extra space [T/F]          ...
Quiz• In Linked allocation it is easy to get the contiguous space  [T/F]           False• In Counting it is easy to get th...
Other subject materials•   Web designing•   Micro processors•   C++ tutorials•   javahome                 http://raj-os.bl...
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Free space managment46

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Free space managment46

  1. 1. Free Space Management 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:• Secondary memory, types of secondary memory• Various types of heads, advantages of one over other• Techniques for increasing storage capacity• Characteristics of the disk http://raj-os.blogspot.in/ 3
  4. 4. ObjectivesOn 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 http://raj-os.blogspot.in/ 4
  5. 5. Understand Free Space Management• Need for free space management – Limited amount of disk space – Necessary to reuse the space from deleted files http://raj-os.blogspot.in/ 5
  6. 6. 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 http://raj-os.blogspot.in/ 6
  7. 7. 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 http://raj-os.blogspot.in/ 7
  8. 8. Various Implementations of Free Space List• Bit Vector• Linked List• Grouping• Counting http://raj-os.blogspot.in/ 8
  9. 9. 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 http://raj-os.blogspot.in/ 9
  10. 10. 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…… http://raj-os.blogspot.in/ 10
  11. 11. Bit Vector (n blocks) 0 1 2 n-1 … 0 ⇒ block[i] free bit[i] = 1 ⇒ block[i] occupiedBlock number calculation (number of bits per word) * (number of 0-value words) + offset of first 1 bit http://raj-os.blogspot.in/ 11
  12. 12. Bit Vector• Bit map requires extra spaceExample: – Disk Size = 1.3 GB – Block size = 512 bytes – Need a bit map of over 332 KB to track its free blocks http://raj-os.blogspot.in/ 12
  13. 13. Bit Vector• Advantage: – Relatively Simple and Efficient – Easy to get contiguous files http://raj-os.blogspot.in/ 13
  14. 14. 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 http://raj-os.blogspot.in/ 14
  15. 15. Linked Free Space List on DiskExample:• 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 http://raj-os.blogspot.in/ 15
  16. 16. Fig 1Linked free space list on disk http://raj-os.blogspot.in/ 16
  17. 17. Linked List• Linked list (free list) – Cannot get contiguous space easily – No waste of space – Not efficient for faster access – Substantial I/O http://raj-os.blogspot.in/ 17
  18. 18. 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 http://raj-os.blogspot.in/ 18
  19. 19. Block No Block No Block NoBlock No Block No Block No . . . ... . . . .Block No Block No Block NoFirst Free block Fig 2 Grouping http://raj-os.blogspot.in/ 19
  20. 20. Grouping• Advantage – Large number of free blocks can be found quickly http://raj-os.blogspot.in/ 20
  21. 21. 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 count http://raj-os.blogspot.in/ 21
  22. 22. Counting• Entry in a free space list consists of – Disk address – Count http://raj-os.blogspot.in/ 22
  23. 23. CountingConstraints: • 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 http://raj-os.blogspot.in/ 23
  24. 24. SummaryIn this class, you have learnt:• What is free space management• Various free space management techniques http://raj-os.blogspot.in/ 24
  25. 25. Frequently asked questions1. Explain the need for free space management2. List the various implementations of free space list3. Explain bit vector http://raj-os.blogspot.in/ 25
  26. 26. Frequently Asked Questions4. Explain the following free space management techniquesa) linked listb) Groupingc) Counting5. Write the advantages and disadvantages of linked list method of free space management http://raj-os.blogspot.in/ 26
  27. 27. Quiz• To keep track of disk space the system maintains Free space list• Bit map requires extra space [T/F] True http://raj-os.blogspot.in/ 27
  28. 28. Quiz• In Linked allocation it is easy to get the contiguous space [T/F] False• In Counting it is easy to get the contiguous space [T/F] True http://raj-os.blogspot.in/ 28
  29. 29. Other subject materials• Web designing• Micro processors• C++ tutorials• javahome http://raj-os.blogspot.in/ 29

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