RE: mdadm raid1 read performance

["Leslie Rhorer" <lrhorer@satx.rr.com>]

> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
> owner@vger.kernel.org] On Behalf Of Keld Jørn Simonsen
> Sent: Friday, May 06, 2011 4:54 PM
> To: Leslie Rhorer
> Cc: 'Keld Jørn Simonsen'; 'NeilBrown'; linux-raid@vger.kernel.org
> Subject: Re: mdadm raid1 read performance
> 
> 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.

	They are certainly not unheard-of, but I'm not sure I would call
them, "commonplace".  They are definitely not in the majority.  I work for a
very large national telecommunications company, and most of the links we
sell are still less than 10M.  I'm not sure we have sold any full 10G
network links, at all, although we have certainly sold a number of 2G - 4G
links.  Of course, WAN and SAN applications are always more expensive than
LAN applications, so many companies have large intra-site links but
comparatively small inter-site links.  Our customer backbone, of course, is
much, much higher than 10G, but none of our internal LAN links at any of our
locations is more than 1G.  Most are 100M.

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