Re: RAID 5 7+1 with oracle

I'm not convinced your interpretation of the graphs is correct.

Left graph p9: Total for 4 disks (2d + 2d)

Writes on the 10K rpm start to hit their catastrophe point at 400 IO/s Writes on the 15K rpm start to hit their catastrophe point at 600 IO/s

Right graph p10: Total of 8 disks (7D + 1P)

Writes on the 10K rpm start to hit their catastrophe point at 400 IO/s Writes on the 15K rpm start to hit their catastrophe point at 600 IO/s

Exactly the same limit despite having twice the spindles.

If you want to compare write TIMES, you should be looking at the right graph on p9: 4D + 4D (eight disks) which get to the catastrophe point at 800 and 1200 IO/S

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

Author: Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html  

> 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.
>

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