Re: mdadm raid1 read performance

From: "Keld Jørn Simonsen" <keld@keldix.com>
To: Leslie Rhorer <lrhorer@satx.rr.com>
Cc: 'John Robinson' <john.robinson@anonymous.org.uk>,
	'Liam Kurmos' <quantum.leaf@gmail.com>,
	'Linux RAID' <linux-raid@vger.kernel.org>
Subject: Re: mdadm raid1 read performance
Date: Fri, 6 May 2011 23:56:26 +0200	[thread overview]
Message-ID: <20110506215626.GB24391@www2.open-std.org> (raw)
In-Reply-To: <09.95.16951.AAD54CD4@cdptpa-omtalb.mail.rr.com>

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