From mboxrd@z Thu Jan 1 00:00:00 1970 From: Keld =?iso-8859-1?Q?J=F8rn?= Simonsen Subject: Re: mdadm raid1 read performance Date: Fri, 6 May 2011 23:53:39 +0200 Message-ID: <20110506215339.GA24391@www2.open-std.org> References: <20110505111006.GB11441@www2.open-std.org> <89.70.16951.22664CD4@cdptpa-omtalb.mail.rr.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: Content-Disposition: inline In-Reply-To: <89.70.16951.22664CD4@cdptpa-omtalb.mail.rr.com> Sender: linux-raid-owner@vger.kernel.org To: Leslie Rhorer Cc: 'Keld =?iso-8859-1?Q?J=F8rn?= Simonsen' , 'NeilBrown' , linux-raid@vger.kernel.org List-Id: linux-raid.ids 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 J=F8rn 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 > >=20 > > On Thu, May 05, 2011 at 09:45:38AM +1000, NeilBrown wrote: > > > On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos > > wrote: > > > > > > > as a separate question, what should be the theoretical performa= nce of > > raid5? > > > > > > x(N-1) > > > > > > So a 4 drive RAID5 should read at 3 time the speed of a single dr= ive. > >=20 > > Actually, theoretically, it should be more than that for reading, m= ore > > like N minus > > some overhead. In a raid5 stripe of 4 disks, when reading you do no= t read > > the checksum block, and thus you should be able to have all 4 drive= s > > 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_1= 0.html > > The latter reports a 3.44 times performance for raid5 reads with 4 > > disks, significantly over the N-1 =3D 3.0 mark. > >=20 > > For writing, you are correct with the N-1 formular. >=20 > 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 sc= ope 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 netwo= rk > actually can make use of such speeds. Of course, if the application > actually obtaining the raw data is on the machine, then network perfo= rmance > is much less of an issue. A database search implemented directly on = the > server, for example, can use every bit of performance available to th= e 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 exces= s of > 120MBps is irrelevant. I mean, yeah, it=92s a rather nice feeling th= at my > RAID arrays can deliver more than 450MBps if they are ever called upo= n 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 a= ll that > much from increased performance? 10 Gbit/s connections are getting commonplace these days, at least in t= he environments that I operate in. Best regards keld -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html