Re: RAID + PostgreSQL?

On Mon, 2006-06-26 at 08:59, Tony Caduto wrote:
> MG wrote:
> > Hello,
> >
> > we are using PostgreSQL 8.0.3 together with RAID on OpenServer 6.
> >
> > When we do a big SELECT-query the whole maschine becomes very very
> > very slowly or stands.
> > The maschine has 3 GB RAM, so we suppose it`s the RAID.
> >
> > Has anyone some experience with RAID + PostgreSQL?
> >
> > Where does PostgreSQL saves his temporary files? Perhaps these are the
> > reason for the bad performance.
> >
> > Greetings
> >
> > Michaela
> What kind of RAID? I know if you have it set up to mirror it becomes
> slow as pond water.

I have to say this has NOT been my experience. With a pair of U320
drives on an LSI-Megaraid with battery backed cache (256M or 512M, not
sure which it was, it's been a few years) our pg server was noticeable
faster with a mirror set than with a single IDE drive (with cache
disabled) on the same machine. And faster than a single SCSI drive with
no RAID controller as well.

> I have a server that was a hand me down so I did not have a choice in
> the RAID and it was set up to mirror with two drives and
> the performance on large selects was very bad.

Wow, something's wrong then. normally, selects are much faster on
mirror sets rather than on a single drive. Do you have a lot of sorts
spilling onto disc?

> The performance would
> increase by 50 or more percent if
> fsync = off is set in the postgresql.conf file.

Of selects? Or performance in general? I can see if for performance in
general, but selects really shouldn't be greatly affected by fsync.

In another vein, I agree with Joshua. There could be LOTS of causes of
poor performance. I wouldn't just assume it's RAID until it's been
proven to be the cause of the problem.