Unanswered: AUTOMATIC Prefetch, DB2_PARALLEL_IO & SANs
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.
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.