RE: Losing data from Postgres

IMHO the biggest problem with RAID 5 is the performance. Under
normal conditions RAID 0+1 have double read performance (since each mirror
can be accessed indepently) and in case of an error things get really bad
for RAID 5.

If a disc fails iin a RAID 5 every read to the failed disc (or disc-stripe),
requires read
from all other discs to reconstruct data, which decreases the bandwidth to
the
discsystem dramatically.
Since most systems do most reads, this is really serious.

On a RAID 0+1 (some times called RAID 10) only the disc that has lost it
mirror
decreases its bandwidth to "normal" single disc speed. All others keep
running
a double speed. And actually write performance increases since the failed
mirror can be updated.

Another point is reliability. If disc number 2 fails in a RIAD 5 we goes
down,
if we eg. have a 2 x 4 RAID 10 system, with a single disc failiur, the
chances
that the second disc fail will cause break down id on 1/7, since 3/4 of the
disc system is still redundant.

Double failiur is not happening often, but has happend if all discs is
installed
at the same time.

If I should set up an important server I will go for RAID 10, mostly because
on a heavy loaded server, the disc error bandwidth decrease in RAID 5 is the
same as
not availably.

/Thomas

> -----Original Message-----
> From: Hossein S. Zadeh [SMTP:hossein(at)hossein(dot)bf(dot)rmit(dot)edu(dot)au]
> Sent: Thursday, November 16, 2000 7:17 AM
> To: admin
> Subject: Re: [ADMIN] Losing data from Postgres
>
> On Wed, 15 Nov 2000, Alfred Perlstein wrote:
>
> > * Serge Canizares <serge(at)ephilosopher(dot)com> [001115 08:23] wrote:
> > >
> > > Of course, if someone sees a reason that RAID 5 would be better than
> RAID 1+0,
> > > I'd appreciate an explanation!
> >
> > Cost. :)
> >
>
> A little bit more explanation: :-)
>
> RAID 1+0 gives you only half of the installed space but gives you (n/2)
> times speed of individual disks. For example for 4 hard disks 1G each, you
> get only 2G of space but double the speed of individual disks.
>
> RAID 5 gives you space equal to (n-1) times individual disks. This is far
> better than RAID 1+0. For example for 4 hard disks 1G each, you get 3G
> space (this is only 25% waste compared to 50% for RAID 1+0). As you add
> hard disks to the array, the 25% ratio of RAID 5 get lower and lower, but
> that of RAID 1+0 stays at 50%. For 10 hard disks for example, the ratio
> gets down to 10%.
> Speed of RAID 5 however is very much dependant of a few factors: speed of
> the controller (or CPU speed in case of software RAID), type of data, and
> how the array is setup (how many blocks of data per strip, etc.). In
> theory, it can exceed speed of RAID 1+0, but I have never seen it in real
> life (but it does approach that of RAID 1+0 if you spend $$$ on the
> controller or CPU).
>
>
> Hope it helps,
> Hossein
>