Re: RAID 5 7+1 with oracle

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_at_jlcomp.demon.co.uk>:

>
> 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_at_structureddata.org>
>> 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.
>>
>
> --
> http://www.freelists.org/webpage/oracle-l
>
>
>
>

Chris Dunscombe

www.christallize.com

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