Topic
3 replies Latest Post - ‏2013-01-10T15:44:42Z by SystemAdmin
SystemAdmin
SystemAdmin
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Pinned topic which is the basic read unit in gpfs, sub block or block?

‏2013-01-10T03:15:19Z |
for example, if the block size is 512k, so sub block size is 512k/32=16k.
when gpfs read a file size which is 32k, will it read only 2 sub block or the entire block contains 2 sub block.

Thanks very much.
Updated on 2013-01-10T15:44:42Z at 2013-01-10T15:44:42Z by SystemAdmin
  • dlmcnabb
    dlmcnabb
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    Re: which is the basic read unit in gpfs, sub block or block?

    ‏2013-01-10T06:25:18Z  in response to SystemAdmin
    When reading sequentially, GPFS will read in fullblocks, or as many subblocks as there are to end of file. In this case it would read the complete 32K.

    When doing random reads it only reads the actual sectors necessary to satisfy the read request. If the first read is at offset 0 of the file, it is assumed to be sequential until proven otherwise by non-sequential reads.
    • SystemAdmin
      SystemAdmin
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      Re: which is the basic read unit in gpfs, sub block or block?

      ‏2013-01-10T14:30:54Z  in response to dlmcnabb
      hi,dlmcnabb.
      i want to verify what you say. how to test it?
      i do not see anything about which is the basic read unit in official gpfs document.
      can you tell how do you know this?
      • SystemAdmin
        SystemAdmin
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        Re: which is the basic read unit in gpfs, sub block or block?

        ‏2013-01-10T15:44:42Z  in response to SystemAdmin
        You could set up a test system and trace/log/watch the scsi commands or trace the kernel disk io driver(s) or similar low level trace/debug ...
        Some SAN attached controllers let you collect stats ...