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RAID5 question, take 2

From: Gary Weber <gweber_at_cji.com>
Date: Sun, 10 Jun 2001 19:03:28 -0700
Message-ID: <F001.003239CA.20010610191520@fatcity.com>

The reply below was a great post! As were replies prior to it. But, none of the replies were for the original question.

The issue in hand is not which raid level to use or whether to use at all.

The question is, and I promise this is the very last time I post it: given 9 hard drives dedicated for RAID5, should data reside on 6 drives via volume group A and indexes on the other 3 drives via volume group B, or should data and indexes be placed on all 9 drives via one volume group? The data is absolutely static.

Gary

> Since RAID5 means that data is striped, of course read performance is OK.
As
> soon as you talk write performance, however, RAID5 becomes something of a
joke
> since it was invented back in the 70's to offer a cheap alternative to the
fast,
> extremely expensive disks offered by IBM back then. So the focus was on
limiting
> the number of disks. Today, where disks in general are cheap and caches
are
> expensive, I really have a hard time figuring out why people buy RAID5
(few
> disks, cache required to compensate for the horrible write penalty)
instead of
> RAID1+0 (more disks, no cache required). And I have a hard time figuring
out why
> the vendors are pushing RAID5 solutions, if RAID1+0 means selling more
disks to
> the customers :-). The answer, of course, is that they are making money on
> caches, not disks.
>
> Technically speaking, RAID1+0 will always be better than RAID5, of course.
Oh,
> they will try to compensate with caches and talk of RAID3 techniques and
what
> have you. RAID1+0 is still superior to RAID5 in any techinal aspect.
>
> It becomes really absurd when you look at the SAN offerings on the market.
For
> instance, IBM's Shark only offers the customer the choice between JBOD
(Just a
> Bunch Of Disks, ie., Non-RAID) and RAID5. IBM has a red book out regarding
this
> and on page 127 out of 228 or so you can read the headline: "JBOD or
RAID5?" and
> that's when it dawns on you that Shark (which is very expensive) cannot
under
> any circumstances be configured for anything else than RAID5 or non-RAID.
> Workaround: Place a file system on top that at least can be striped
(Veritas,
> for instance).
>
> EMC has a standard offering where they'll suggest RAID-S (S looks a lot
like 5,
> doesn't it?) and the standard answer if write performance is not good
enough is:
> "Add more cache". Well, we had a customer who reached 32 GB of cache (not
MB,
> mind you, but GB) and write performance was still bad (of course) for
restores
> and recovery operations and file copying and all those things where a
cache can
> never help you. Fortunately, EMC can be re-configured for RAID1+0, which
the
> customer finally did, and all went well. They could then return the
expensive
> cache and save some money :).
>
> Same problem with HP (Hitachi) - they'll try to pursuade you to buy a very
> expensive RAID5 system. It's like trying to talk you into paying a lot of
money
> for a WWII Spitfire, claiming that the avionics have been upgraded a great
deal
> and that for the general user, this is much better than todays aircraft
:-))).
>
> We have lots of horror stories like this regarding RAID5. Of course it's
good
> enough in a lot of situations. But you should know the reason why it's
good
> enough. And the moment you have to restore or recover anything, you will
> discover the true price (factor 4, usually) of RAID5, namely the write
penalty.
> No amount of cache can help you in those situations.
>
> Christopher Spence wrote:
>
> > Static data raid 5 is a very good option, it has great read performance
and
> > very inexpensive.
> >
> > "Walking on water and developing software from a specification are easy
if
> > both are frozen."
> >
> > Christopher R. Spence
> > Oracle DBA
> > Fuelspot

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-- 
Author: Gary Weber
  INET: gweber_at_cji.com

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Received on Sun Jun 10 2001 - 21:03:28 CDT

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