Re: Postgresql on software RAID

On Tue, 16 Dec 2003 sumbry@sumbry.com wrote:

> > I am experimenting with a few OS's for my new hardware. I plan to have a
> > software RAID5 device for my pgsql data directory.
> >
> > I have been experimenting with FreeBSD and with Linux, does anyone have any
> > thoughts on whether vinum RAID devices are better than Linux software RAID?
> > Or vice versa... Or no difference at all? Any gotchas I should bear in mind?
>
> I use Postgres w/Vinum setup to mirror 2 9gig SCSI Cheetahs and I have
> no complaints.  I bang on the db quite a bit (moderately busy site and
> tons of data analysis) and performance is great.  Ultimately I am going to
> slap a hardware RAID card in there (need the immediate failover) but right
> now software wise performance is great.  I'd probably only recommend
> doing mirroring in software though, RAID-5 is going to eat up alot more
> CPU and disk writes.

Just a clarification, it only costs for writes.  For reads, an N disk
RAID5 is exactly as fast as an N-1 disk RAID0.  On a modern fast CPU
machine, with a small write load, CPU usage is usually very low, say <5%.

That said, my production databases usually sit on hardware RAID5 with
battery backed cache, build / test servers often run on software RAID5.
Money, meet mouth.  :-)