Re: Help with low raid5 performance

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Tue, Jan 11, 2011 at 09:34:27AM +0000, Stephen Borrill wrote:
>
> >>I'm using a 4-disk RAID 5 with the partition starting at 45416448
> >>(which is very well aligned (on a 65536 boundary!).

This should not be aligned on a 64k boundary with 3 data disks.  It
should be aligned on a 48K boundary!

Greg, with RAIDframe sector 0 should always be at the beginning of a
stripe, so 48K alignment with no remainder is correct, yes?

> Changing to 32 1 1 5 gave:
>       -------Sequential Output-------- ---Sequential Input-- --Random--
>       -Per Char- --Block--- -Rewrite-- -Per Char- --Block--- --Seeks---
>    MB K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU K/sec %CPU  /sec %CPU
>  2048  9137  9.6  7440  3.3  3045  1.3 61801 82.3 149558 37.1 228.2  2.4
> 
> These were formatted with: newfs -O 2 -f 4096 -b 32768

This newfs command line is wrong.  32 doesn't go evenly into 48.  Remember
that the RAIDframe sectors-per-stripe is in half-kilobyte units...

You probably want newfs -f 2048 -b 16384 -a 3.

Thor