Re: mdadm raid1 read performance

["Keld Jørn Simonsen" <keld@keldix.com>]

On Fri, May 06, 2011 at 04:20:39PM -0500, Leslie Rhorer wrote:
> > -----Original Message-----
> > From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
> > owner@vger.kernel.org] On Behalf Of Keld Jørn Simonsen
> > Sent: Thursday, May 05, 2011 6:10 AM
> > To: NeilBrown
> > Cc: Liam Kurmos; Roberto Spadim; Brad Campbell; Drew; linux-
> > raid@vger.kernel.org
> > Subject: Re: mdadm raid1 read performance
> > 
> > On Thu, May 05, 2011 at 09:45:38AM +1000, NeilBrown wrote:
> > > On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos <quantum.leaf@gmail.com>
> > wrote:
> > >
> > > > as a separate question, what should be the theoretical performance of
> > raid5?
> > >
> > > x(N-1)
> > >
> > > So a 4 drive RAID5 should read at 3 time the speed of a single drive.
> > 
> > Actually, theoretically, it should be more than that for reading, more
> > like N minus
> > some overhead. In a raid5 stripe of 4 disks, when reading you do not read
> > the checksum block, and thus you should be able to have all 4 drives
> > occupied with reading real data. Some benchmarks back this up,
> > http://home.comcast.net/~jpiszcz/20080329-raid/
> > http://blog.jamponi.net/2008/07/raid56-and-10-benchmarks-on-26255_10.html
> > The latter reports a 3.44 times performance for raid5 reads with 4
> > disks, significantly over the N-1 = 3.0 mark.
> > 
> > For writing, you are correct with the N-1 formular.
> 
> 	There have been a lot of threads here about array performance, but
> one important factor rarely mentioned in these threads is network
> performance.  Of course, network performance is really outside the scope of
> this list, but I frequently see people talking about performance well in
> excess of 120MBps.  That's great, but I have to wonder if their network
> actually can make use of such speeds.  Of course, if the application
> actually obtaining the raw data is on the machine, then network performance
> is much less of an issue.  A database search implemented directly on the
> server, for example, can use every bit of performance available to the local
> machine.  Given that in my case the vast majority of data is squirted across
> the LAN (e.g., these are mostly file servers), anything much in excess of
> 120MBps is irrelevant.  I mean, yeah, it’s a rather nice feeling that my
> RAID arrays can deliver more than 450MBps if they are ever called upon to do
> so, but with a 1G LAN, that's not going to happen very often.  I just wonder
> how many people who complain of poor performance can really benefit all that
> much from increased performance?

10 Gbit/s connections are getting commonplace these days, at least in the
environments that I operate in.

Best regards
keld
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