On Wed, 15 Sep 2004 09:11:37 -0700, Jeffrey W. Baker <jwbaker@acm.org> wrote:
> On Wed, 2004-09-15 at 02:39, Michael Paesold wrote:
> > Jeffrey W. Baker wrote:
> >
> > > Current issue:
> > >
> > > A dual 64-bit Opteron 244 machine with 8GB main memory, two 4-disk RAID5
> > > arrays (one for database, one for xlogs). PG's config is extremely
> > > generous, and in isolated benchmarks it's very fast.
> >
> > It depends on the controller, but usually I would expect a better
> > performance if xlogs are just on a two-disk mirror and the rest of the disks
> > for data (6 splindles instead of 4 then).
> >
> > I don't think RAID5 is a benefit for xlogs.
>
> All these replies are really interesting, but the point is not that my
> RAIDs are too slow, or that my CPUs are too slow. My point is that, for
> long stretches of time, by database doesn't come anywhere near using the
> capacity of the hardware. And I think that's odd and would like to
> config it to "false".
Umh, I don't think you have shown any numbers to show if the database
is using the capacity of the hardware or not...
If this is a seek heavy operation, the raw throughput is irrelevant;
you are limited by the number of seeks your disks can do. Run some
iostats and look at the number of transactions per second.
Using raid 5 can just destroy the number of write transactions per
second you can do, especially if it is software raid or a cheap raid
controller.
You can't just say "the hardware is fine and not stressed so I don't
want to discuss that, but everything is too slow so please make it
faster".