On Thu, Apr 29, 2010 at 1:51 PM, Scott Whitney <scott@xxxxxxxxxxx> wrote: > During the testing that I did when moving from pg7 to pg8 a few years back, I didn't notice any particular performance > increase on a similarly-configured server. > > That is, we've got 14 disks (15k rpm) striped in a single RAID10 array. Moving the logs to an internal RAID > versus leaving them on the "main" storage array didn't impact my performance noticeably either way. > > Now, please note: > SUCH A STATEMENT DEPENDS ENTIRELY ON YOUR USE-CASE SCENARIO. > > That is, because these tests which I performed showed that for me, if you're using PG in a much different > manner, you may have different results. It's also quite possible that our 50% expansion in the past 3 > years has had some effect, but I'm not in a place to retest that at this time. > > We specifically chose to put our logs on the fiber SAN in case the underlying machine went down. > Disaster recovery for that box would therefore be: > a) New machine with O/S and pg installed. > b) Mount SAN > c) Start PG. Everything (including logs) is available to you. > > It is, in essence, our "externally-stored PG data" in its entirety. > > On the 10k vs 15k rpm disks, there's a _lot_ to be said about that. I don't want to start a flame war here, > but 15k versus 10k rpm hard drives does NOT equivocate to a 50% increase in read/write times, to say > the VERY least. > > "Average seek time" is the time it takes for the head to move from random place A to > random place B on the drive. The rotational latency of a drive can be easily calculated. > A 15k drive rotates roughly 250 times per second, or 4 msec per rotation versus a 10k > drive which is about 167 rotations per sec or 6 sec per rotation. > > This would mean that the rotational latency of a 15k drive adds 2msec and a 10k drive adds 3msec. > > So, your true seek time is the "average seek time" of the drive + the rotation listed above. > > So, if your average latency is something REALLY good (say 4msec) for each of the drives, your 15k > drive would have 6msec real-world IOPS of around 166, and your 10k drive would have 143. In that > particular case, at a very low level, you'd be getting about a 14% improvement. > > HOWEVER, we're not talking about a single drive, here. We're talking about a RAID10 of 12 > drives (6 + 6 mirror, I assume) versus 24 drives (12+12 mirror I assume). In that case, > the max IOPS of your first RAID would be around 1000 while the max IOPS of your second RAID > with the "slower" drives would be around 1700. > > > Hope this helps. > > I _really_ don't want to start a war with this. If you're confused how I got these > numbers, please contact me directly. No argument from me. My point was that the RAID-10 is for random access at x op/s while the WAL is for sequential IO at x mb/s. If you put the pg_xlog on the same drives as the data set, then you basically halve throughput by writing everything randomly twice. Better RAID controllers with lots of cache can re-order all that so that you still get good throughput. If you've got a RAID controller with less memory, then it can really help to have a separate pg_xlog. My machines about 30% or so faster than with it all on one set with an areca 1680. -- Sent via pgsql-admin mailing list (pgsql-admin@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-admin
