On Fri, May 06, 2011 at 03:44:31PM -0500, Leslie Rhorer wrote:

quoted

-----Original Message-----
From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
owner@vger.kernel.org] On Behalf Of John Robinson
Sent: Tuesday, May 03, 2011 7:57 PM
To: Liam Kurmos
Cc: Linux RAID
Subject: Re: mdadm raid1 read performance

On 04/05/2011 01:07, Liam Kurmos wrote:

quoted

Hi,

I've been testing mdadm (great piece of software btw) however all my
test show that reading from raid1 is only the same speed as reading
from a single drive.

Is this a known issue? or is there something seriously wrong with my
system? i have tried v2.8.1 and v.3.2.1 without difference and several
benchmarking methods.

This is a FAQ. Yes, this is known. No, it's not an issue, it's by design
- pretty much any RAID 1 implementation will be the same because of the
nature of spinning discs. md RAID 1 will serve multiple simultaneous
reads from the different mirrors, giving a higher total throughput, but
a single-threaded read will read from only one. If you want RAID 0
sequential speed at the same time as RAID 1 mirroring, look at md RAID
10, and in particular RAID 10,f2; please see the excellent documentation
and wiki for more details.

	I would go so far as to say it is more than just by design.  It is
by the very fundamental nature of RAID1.  RAID1 is intended to be a simple
mirror.  Every write is sent in identical form to precisely the same logical
sector of all devices.  Any read can come from any device in the array.  The
WriteMostly specifier can help insure the best throughput in the case where
one of the members is inherently slower than the other members of the array,
and some RAID1 implementations support load balancing, but otherwise there
are no real operational gains in performance for a RAID1 array over a single
disk.

SNIA defines RAID1 variants that are not so simple. And in many cases
you really do not know the internal layout of HW RAID1. So IMHO what you say does
not hold true.

best regards
keld