Re: RAID 5 7+1 with oracle
- From: Chris Dunscombe <chris@xxxxxxxxxxxxxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Fri, 05 Oct 2007 13:58:57 +0100
Jonathan,
Just a couple of points on the reads for a mirrored configuration.
I know it seems obvious that it should read from both primary and the
mirror but
about 5 years ago when I was at a site considering an HP XP256 I was
told by the
HP pre-sales guy that when configured as mirror as opposed to RAID-5 then NO
reads would be satisfied by the mirror. Surprising I know.
In this case it obviously does read from the mirror but it maybe purely
alternating the requests from the primary and then the mirror rather
than than
operating a pure single queue multi-server approach.
Cheers,
Chris
Quoting Jonathan Lewis <jonathan@xxxxxxxxxxxxxxxxxx>:
The same applies with read TIMES - surely you should be comparing
the two right-hand graphs. (I'm a little surprised by the similarity of the
graphs for reads - in a simplistc queueing theory model I would approximate
the RAID-10 as 4 x M/M/2, and the RAID-5 as 8 * M/M/1 with 7/8ths
load - and expected the M/M/2 to outperform the other). But maybe it's not
really valid to call RAID10 M/M/2; maybe hp just have some dirty
tricks (or errors) in their algorithms that even things out.
If you want to compare volume of data stored, of course, that's a completely
different matter.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
Date: Thu, 4 Oct 2007 17:35:14 -0700
From: "Greg Rahn" <greg@xxxxxxxxxxxxxxxxxx>
Subject: Re: RAID 5 7+1 with oracle
In order to avoid a religious war I'll just offer this:
Look at:
http://h71028.www7.hp.com/ERC/downloads/4AA0-7923ENW.pdf
This document shows that RAID5 (7D+1P : page 10 right graph) is far
better than RAID 1(2D+2D : page 9 left graph) both on read and write
operation.
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Chris Dunscombe
www.christallize.com
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