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@xxxxxxxxxx>
To: <oracle-l@xxxxxxxxxxxxx>
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 *

Upcoming events:
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-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx =
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Jared Still
Sent: Tuesday, December 14, 2004 3:58 PM
To: chris@xxxxxxxxxxxxxxxxxxxxx
Cc: Stephen.Lee@xxxxxxxx; oracle-l@xxxxxxxxxxxxx
Subject: Re: Storage array advice anyone?

On Tue, 14 Dec 2004 10:47:20 +0000, chris@xxxxxxxxxxxxxxxxxxxxx=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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