Results 1 to 4 of 4
  1. #1
    Join Date
    Jul 2004

    Unanswered: AUTOMATIC Prefetch, DB2_PARALLEL_IO & SANs

    Hey all,

    Trying to figure out what the deal is with these parms when using a SAN.

    I've got a a RAID5 SAN, each LUN is comprised of 12 disks. But AIX only sees this as 1 disk.
    I've set DB2_PARALLEL_IO=* and I've altered my tablespaces to AUTOMATIC prefetch.

    I'm wondering if DB2 can figure out the number of underlying physical disks or where it gets the number from.
    Basically one of my production systems seems to see 1 disk (so the prefetch on a TABLESPACE snapshot = the extent) and on a test system it's multiplying by 6!

    I'm not too well versed in SAN stuff, but I can find out information from the admin.

    Any ideas?

  2. #2
    Join Date
    Jul 2004
    No ideas guys?

  3. #3
    Join Date
    Sep 2003
    As angels seem to be fearing to tread, let me rush in and say that as far as I know, SAN makes LUNS appear like local physical disks. Your OS sends signals to these LUNS(SAN) as though they are local physical disks. DMS tablespaces, if they contain multiple files(or devices) spread the ( table etc.) extents in a roundrobin fashion. If db2 is doing a full table(or index) scan, one request by db2 to get the next extent translates into getting the next extent from all files of tablespace at the OS level if PARALLEL_IO is on. At the OS level OS parameters minpgahead and maxpgahead (and its JFS2 equivalents) come to play and they must be adjusted to be in Sync.I am interested in knowing how many files there are in prod and test for the tablespace you mention.

  4. #4
    Join Date
    Aug 2006
    Automatic prefetch would mean parallelism of the tablespace is equal to prefetch size divided by the extent size of the tablespace.

    Prefetch requests are broken down into several number smaller requests based on parallelism of tablespace

    with parallelism set to * it will use 6 spindles per container as well as using the I/O of the tablespace

    to optimize RAID Make the EXTENTSIZE of the table space equal to, or a multiple of, the RAID stripe size

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts