SSM
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SSM Presentation Transcript

  • 1. Secondary Storage Management 1
  • 2. Secondary Storage Management “Portion”: a group of contiguous blocks File Allocation Table (FAT) Keeps track of portions assigned to a file Must also keep track of the space available for new allocations 2
  • 3. Secondary Storage Management “Preallocation” Requires maximum size for the file to be known at the time of file creation Facilitates allocation of entire file as one portion on the same cylinder Difficult to reliably estimate the maximum potential size of the file File size is usually overestimated so as to not run out of space “Dynamic allocation” Allocates new portions to a file as needed 3
  • 4. File allocation methods Contiguous allocation One portion is used For each file, the FAT contains the starting block and the number of blocks Requires preallocation Gives great performance for sequential access For “random” access, it is easy to calculate the disk location External fragmentation will occur Need to perform compaction  Note the memory analogy with dynamic partitioning  4
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  • 7. File allocation methods Chained allocation Allocation on basis of individual blocks Each block contains a pointer to the next block in the chain FAT is same as for contiguous allocation Starting block and number of blocks  No external fragmentation Only sequential access is possible with poor performance No accommodation of the principle of locality 7
  • 8. 8
  • 9. “Consolidation” makes sequential access more efficient and accommodates the principle of locality 9
  • 10. File allocation methods Indexed allocation For each file, the FAT contains a separate one-level index table The index table has one entry for each portion allocated to the file Entry consists of the starting block and number of blocks The index table is usually on disk 10
  • 11. File allocation methods Indexed allocation Allocation can be by . . . single blocks  • 1-block portions • no fragmentation variable-sized portions  • better performance Popular method for both sequential and indexed access 11
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  • 13. 13
  • 14. Free space management A disk allocation table records which blocks are available Implementation techniques are as follows . . . Bit table One bit per block Compact representation – can be kept in RAM Easy to find contiguous blocks 14
  • 15. Free space management Chained free portions File of chained portions Only need the location and size of the first portion Indexing One entry for each free portion Treats free space as a file of variable-sized portions 15