On Sun, Aug 8, 2010 at 12:46 AM, Scott Carey <sc...@richrelevance.com> wrote: > > On Aug 5, 2010, at 4:09 PM, Scott Marlowe wrote: > >> On Thu, Aug 5, 2010 at 4:27 PM, Pierre C <li...@peufeu.com> wrote: >>> >>>> 1) Should I switch to RAID 10 for performance? I see things like "RAID 5 >>>> is bad for a DB" and "RAID 5 is slow with <= 6 drives" but I see little on >>>> RAID 6. >>> >>> As others said, RAID6 is RAID5 + a hot spare. >>> >>> Basically when you UPDATE a row, at some point postgres will write the page >>> which contains that row. >>> >>> RAID10 : write the page to all mirrors. >>> RAID5/6 : write the page to the relevant disk. Read the corresponding page >>> from all disks (minus one), compute parity, write parity. >> >> Actually it's not quite that bad. You only have to read from two >> disks, the data disk and the parity disk, then compute new parity and >> write to both disks. Still 2 reads / 2 writes for every write. >> >>> As you can see one small write will need to hog all drives in the array. >>> RAID5/6 performance for small random writes is really, really bad. >>> >>> Databases like RAID10 for reads too because when you need some random data >>> you can get it from any of the mirrors, so you get increased parallelism on >>> reads too. >> >> Also for sequential access RAID-10 can read both drives in a pair >> interleaved so you get 50% of the data you need from each drive and >> double the read rate there. This is even true for linux software md >> RAID. > > > My experience is that it is ONLY true for software RAID and ZFS. Most > hardware raid controllers read both mirrors and validate that the data is > equal, and thus writing is about as fast as read. Tested with Adaptec, > 3Ware, Dell PERC 4/5/6, and LSI MegaRaid hardware wise. In all cases it was > clear that the hardware raid was not using data from the two mirrors to > improve read performance for sequential or random I/O.
Interesting. I'm using an Areca, I'll have to run some tests and see if a mirror is reading at > 100% read speed of a single drive or not. -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
