Record-Blocking

Record Blocking

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Blocks are the unit of I/O for secondary storage Blocks are the unit of I/O for secondary storage
Records are logical unit of access, and must be Records are logical unit of access, and must be organized in blocks to perform I/O organized in blocks to perform I/O
Three methods:

  • Fixed blocking
    •  Fixed length records are used, with inegral number of records stored in a block. Internal fragmentation stored in a block. Internal fragmentation
  • Variable-length spanned blocking
    • Variable-length records are used, packed into blocks with no unused space. Pointers used to span blocks unused space. Pointers used to span blocks
  • Variable-length unspanned blocking
    • Same as above without spanning, with wasted space in most Same as above without spanning, with wasted space in most blocks, because of inability to use remainders blocks

Record Blocking

  • Fixed blocking common for sequential files with fixed -length records length records
  • Variable -length spanned blocking is efficient of length  is efficient of storage and does not limit record size,  but more complicated to implement and sometimes complicated to implement and sometimes inefficient. Files are more difficult to update
  • Variable -length unspanned blocking results in blocking results in wasted space and limits record size to the size of  the block
  • Record
    • -blocking technique may interact with VM. blocking technique may interact with VM.
    • Page may be implemented as integral number of Page may be implemented as integral number of
    • blocks, or vice versa

References

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  • WikiNote Foundation

Last modified: Friday, 20 September 2019, 3:23 PM