Re: RAID + PostgreSQL?

[Scott Marlowe <smarlowe(at)g2switchworks(dot)com>]

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.