On Sat, Aug 29, 2009 at 9:59 AM, Scott Marlowe<scott.marlowe@xxxxxxxxx> wrote: > On Sat, Aug 29, 2009 at 2:46 AM, Greg Stark<gsstark@xxxxxxx> wrote: >> On Sat, Aug 29, 2009 at 5:20 AM, Luke Koops<luke.koops@xxxxxxxxxxx> wrote: >>> Joseph S Wrote >>>> If I have 14 drives in a RAID 10 to split between data tables >>>> and indexes what would be the best way to allocate the drives >>>> for performance? >>> >>> RAID-5 can be much faster than RAID-10 for random reads and writes. It is much slower than RAID-10 for sequential writes, but about the same for sequential reads. For typical access patterns, I would put the data and indexes on RAID-5 unless you expect there to be lots of sequential scans. >> >> That's pretty much exactly backwards. RAID-5 will at best slightly >> slower than RAID-0 or RAID-10 for sequential reads or random reads. >> For sequential writes it performs *terribly*, especially for random >> writes. The only write pattern where it performs ok sometimes is >> sequential writes of large chunks. > > Note that while RAID-10 is theoretically always better than RAID-5, > I've run into quite a few cheapie controllers that were heavily > optimised for RAID-5 and de-optimised for RAID-10. However, if it's > got battery backed cache and can run in JBOD mode, linux software > RAID-10 or hybrid RAID-1 in hardware RAID-0 in software will almost > always beat hardware RAID-5 on the same controller. raid 5 can outperform raid 10 on sequential writes in theory. if you are writing 100mb of actual data on, say, a 8 drive array, the raid 10 system has to write 200mb data and the raid 5 system has to write 100 * (8/7) or about 114mb. Of course, the raid 5 system has to do parity, etc. For random writes, raid 5 has to write a minimum of two drives, the data being written and parity. Raid 10 also has to write two drives minimum. A lot of people think parity is a big deal in terms of raid 5 performance penalty, but I don't -- relative to the what's going on in the drive, xor calculation costs (one of the fastest operations in computing) are basically zero, and off-lined if you have a hardware raid controller. I bet part of the problem with raid 5 is actually contention. since your write to a stripe can conflict with other writes to a different stripe. The other problem with raid 5 that I see is that you don't get very much extra protection -- it's pretty scary doing a rebuild even with a hot spare (and then you should probably be doing raid 6). On read performance RAID 10 wins all day long because more drives can be involved. merlin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
