> David Lang Wrote: > > with only four drives the space difference between raid 1+0 and raid 5 > isn't that much, but when you do a write you must write to two drives (the > drive holding the data you are changing, and the drive that holds the > parity data for that stripe, possibly needing to read the old parity data > first, resulting in stalling for seek/read/calculate/seek/write since > the drive moves on after the read), when you read you must read _all_ > drives in the set to check the data integrity. Thanks for the explanation David. It's good to know not only what but also why. Still I wonder why reads do hit all drives. Shouldn't only 2 disks be read: the one with the data and the parity disk? > > for seek heavy workloads (which almost every database application is) the > extra seeks involved can be murder on your performance. if your workload > is large sequential reads/writes, and you can let the OS buffer things for > you, the performance of raid 5 is much better. Well, actually most of my application involves large sequential reads/writes. The memory available for buffering (4GB) isn't bad either, at least for my scenario. On the other hand I have got such strong posts against RAID 5 that I doubt to even consider it. > > Linux software raid can do more then two disks in a mirror, so you may be > able to get the added protection with raid 1 sets (again, probably not > relavent to four drives), although there were bugs in this within the last > six months or so, so you need to be sure your kernel is new enough to have > the fix. > Well, here rises another doubt. Should I go for a single RAID 1+0 storing OS + Data + WAL files or will I be better off with two RAID 1 separating data from OS + Wal files? > now, if you can afford solid-state drives which don't have noticable seek > times, things are completely different ;-) Ha, sadly budget is very tight. :) Regards, Fernando. ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
