Re: Storage array advice anyone?

As Cary says, the probabilities have been worked out. But people don't tend to internalize probabilities (at least accurately). They tend to think either "lucky" or "unlucky". I personally like probabilities, but I generally tell managers my experience. In the last 7 years, at least 4 systems I had some contact with (clients/employers/former employers/whatever) had 2 disks fail in close time proximity (within roughly 30 minutes). Calculating the implicit probabilites of these real-life occurrences is way over my head (some were small shops with 1-2 servers, some larger). But I can safely say that, in human terms, such an event is not "unbelievably unlikely".

The likelihood of serious data loss from 2 failed drives is much lower with RAID 10 than with RAID 5. So you've got a performance benefit and a fault tolerance benefit with RAID 10. And I won't even detail the crippling effect I've seen when a RAID 5 disk failed and the system rebuilt it with the hot spare.

--Terry

• Original Message ----- From: "Cary Millsap" <cary.millsap_at_hotsos.com> To: <oracle-l_at_freelists.org> Sent: Tuesday, December 14, 2004 2:38 PM Subject: RE: Storage array advice anyone?

The probabilities are already worked out, and they're publicly available = in
the paper called "RAID: High-Performance, Reliable Secondary Storage" = (an
ACM Surveys article) by Messrs. Chen, Lee, Gibson, Katz, and Patterson.

Not many people bother to put them into Excel, but when I once played = with
the numbers a bit, I realized pretty quickly that the probability of an outage-causing double-whammy is a lot worse than most people think. The article mentions that point specifically, if I remember correctly.

The key idea is that the failures of two disks in an array are = frequently
not independent events. Often, the event that just screwed up disk #1 = has a
higher probability now of screwing up disk #2 before you can fix #1.

Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
* Nullius in verba *

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-----Original Message-----
From: oracle-l-bounce_at_freelists.org =
[mailto:oracle-l-bounce_at_freelists.org]
On Behalf Of Jared Still
Sent: Tuesday, December 14, 2004 3:58 PM To: chris_at_thedunscombes.f2s.com
Cc: Stephen.Lee_at_dtag.com; oracle-l_at_freelists.org Subject: Re: Storage array advice anyone?

On Tue, 14 Dec 2004 10:47:20 +0000, chris_at_thedunscombes.f2s.com=20
> My experience is that with either RAID 5 or 10 you have to be =
unbelievably
> unlucky to lose data providing disks are replaced when they fail and =
not
left
> for a few days or even more. You are talking extremely remote. It =
might be
an
> idea to get someone to do the maths and work out the probabilities.

I, for one, have been that unlucky on at least one occasion.=20

--=20
Jared Still
Certifiable Oracle DBA and Part Time Perl Evangelist

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