Re: Postgres on RAID5

Actualy my statistics were off a bit I realised - chance of failure
for one drive is 1 in X. change of failure in RAID 0 is 7 in X,
chance of one drive failure in 14 drive RAID 5 is 14 in X,13 in X for
second drive, total probably is 182 in X*X, which is much lower than
RAID 0.

Your drive performance is less than stellar for a 14 drive stripe, and
CPU usage for writes is very high. Even so - this should be enough
through put to get over 100 rows/sec assuming you have virtualy no
stored procs (I have noticed that stored procs in plpgsql REALLY slow
pg_sql down).

Alex Turner
netEconomist

On Mon, 14 Mar 2005 15:54:34 -0500, Arshavir Grigorian <ag(at)m-cam(dot)com> wrote:
> Alex Turner wrote:
> > I would recommend running a bonnie++ benchmark on your array to see if
> > it's the array/controller/raid being crap, or wether it's postgres. I
> > have had some very surprising results from arrays that theoretically
> > should be fast, but turned out to be very slow.
> >
> > I would also seriously have to recommend against a 14 drive RAID 5!
> > This is statisticaly as likely to fail as a 7 drive RAID 0 (not
> > counting the spare, but rebuiling a spare is very hard on existing
> > drives).
>
> Thanks for the reply.
>
> Here are the results of the bonnie test on my array:
>
> ./bonnie -s 10000 -d . > oo 2>&1
> File './Bonnie.23736', size: 10485760000
> Writing with putc()...done
> Rewriting...done
> Writing intelligently...done
> Reading with getc()...done
> Reading intelligently...done
> Seeker 1...Seeker 2...Seeker 3...start 'em...done...done...done...
> -------Sequential Output-------- ---Sequential Input-- --Random--
> -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
> MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU /sec %CPU
> 10000 4762 96.0 46140 78.8 31180 61.0 3810 99.9 71586 67.7 411.8 13.1
>
> On a different note, I am not sure how the probability of RAID5 over 15
> disks failing is the same as that of a RAID0 array over 7 disks. RAID5
> can operate in a degraded mode (14 disks - 1 bad), RAID0 on the other
> hand cannot operate on 6 disks (6 disks - 1 bad). Am I missing something?
>
> Are you saying running RAID0 on a set of 2 RAID1 arrays of 7 each? That
> would work fine, except I cannot afford to "loose" that much space.
>
> Care to comment on these numbers? Thanks.
>
>
> Arshavir
>