On Thu, 2006-08-24 at 09:21 -0400, Merlin Moncure wrote: > On 8/22/06, Jeff Davis <pgsql@xxxxxxxxxxx> wrote: > > On Tue, 2006-08-22 at 17:56 -0400, Bucky Jordan wrote: > > Very interesting. I always hear that people avoid RAID 5 on database > > servers, but I suppose it always depends. Is the parity calculation > > something that may increase commit latency vs. a RAID 10? That's > > normally the explanation that I get. > > it's not the parity, it's the seeking. Raid 5 gives you great > sequential i/o but random is often not much better than a single > drive. Actually it's the '1' in raid 10 that plays the biggest role > in optimizing seeks on an ideal raid controller. Calculating parity > was boring 20 years ago as it inolves one of the fastest operations in > computing, namely xor. :) > Here's the explanation I got: If you do a write on RAID 5 to something that is not in the RAID controllers cache, it needs to do a read first in order to properly recalculate the parity for the write. However, I'm sure they try to avoid this by leaving the write in the battery-backed cache until it's more convenient to do the read, or maybe until the rest of the stripe is written in which case it doesn't need to do the read. I am not sure the actual end effect. > > > Lastly, re your question on putting the WAL on the RAID10- I currently > > > have the box setup as RAID5x6 with the WAL and PGDATA all on the same > > > raidset. I haven't had the chance to do extensive tests, but from > > > previous readings, I gather that if you have write-back enabled on the > > > RAID, it should be ok (which it is in my case). > > with 6 relatively small disks I think single raid 10 volume is the > best bet. however above 6 dedicated wal is usually worth considering. > since wal storage requirements are so small, it's becoming affordable > to look at solid state for the wal. > I've often wondered about that. To a certain degree, that's the same effect as just having a bigger battery-backed cache, right? Regards, Jeff Davis
