Topic
4 replies Latest Post - ‏2009-05-01T23:23:04Z by SystemAdmin
SystemAdmin
SystemAdmin
101 Posts
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Pinned topic LRECL and BLKSIZE

‏2009-01-25T10:31:47Z |
hi everybody,

Can some tell me what are the differences between LRECL (Logical Record Length) and the BlockSize (BLKSIZE)?

Marc
Updated on 2009-05-01T23:23:04Z at 2009-05-01T23:23:04Z by SystemAdmin
  • PedroV
    PedroV
    15 Posts
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    Re: LRECL and BLKSIZE

    ‏2009-01-25T22:53:43Z  in response to SystemAdmin
    The record length is fairly obvious. Applications are usually written to expect certain record sizes. This the logicial length, however...

    The records can be grouped together into blocks (physical records) to better utilize the disk storage. Otherwise, the operating system puts control information between records. There is control information used by each block, so the bigger the block size, the less overall control information is used on the disk. When reading, the operating system reads the physical records and feeds logical records to the application program.

    This blocks and records scheme was invented when disk storage was very expensive. The scheme remains, but with the low cost of storage, it is no longer so critical. Specify a blocksize of zero and the operating system will figure the best blocksize.
    • SystemAdmin
      SystemAdmin
      101 Posts
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      Re: LRECL and BLKSIZE

      ‏2009-01-26T16:30:57Z  in response to PedroV
      hi,

      Many thanks for your answer.

      Marc Yves
    • SystemAdmin
      SystemAdmin
      101 Posts
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      Re: LRECL and BLKSIZE

      ‏2009-02-18T10:12:25Z  in response to PedroV
      This is only the half truth. Disk and track utilization is important. But more important is the I/O performance. Because a block ist the transfer unit between memory and DASD unblocked processing would be very slow because each logical record causes an I/O operation which is very expensive. So if the data is blocked with lets say factor hundred, with each I/O operation 100 records are transfered, saving 99 I/Os.
      Wolfram
      • SystemAdmin
        SystemAdmin
        101 Posts
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        Re: LRECL and BLKSIZE

        ‏2009-05-01T23:23:04Z  in response to SystemAdmin
        Just to be clear; PedroV's post was not advocating unblocked records. Unblocked records are indeed very inefficient for I/O. He was advocating using BLKSIZE=0 to let the system determine the optimum blocksize.