From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Leslie Rhorer" Subject: RE: mdadm raid1 read performance Date: Fri, 6 May 2011 22:17:26 -0500 Message-ID: References: <20110506215339.GA24391@www2.open-std.org> Reply-To: Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <20110506215339.GA24391@www2.open-std.org> Sender: linux-raid-owner@vger.kernel.org To: =?iso-8859-1?Q?'Keld_J=F8rn_Simonsen'?= Cc: 'NeilBrown' , linux-raid@vger.kernel.org List-Id: linux-raid.ids > -----Original Message----- > From: linux-raid-owner@vger.kernel.org [mailto:linux-raid- > owner@vger.kernel.org] On Behalf Of Keld J=F8rn Simonsen > Sent: Friday, May 06, 2011 4:54 PM > To: Leslie Rhorer > Cc: 'Keld J=F8rn Simonsen'; 'NeilBrown'; linux-raid@vger.kernel.org > Subject: Re: mdadm raid1 read performance >=20 > 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 > > > > > > 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 perfor= mance > 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 dri= ves > > > 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 =3D 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 we= ll in > > excess of 120MBps. That's great, but I have to wonder if their net= work > > actually can make use of such speeds. Of course, if the applicatio= n > > actually obtaining the raw data is on the machine, then network > performance > > is much less of an issue. A database search implemented directly o= n 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 squirt= ed > across > > the LAN (e.g., these are mostly file servers), anything much in exc= ess > of > > 120MBps is irrelevant. I mean, yeah, it=92s a rather nice feeling = that my > > RAID arrays can deliver more than 450MBps if they are ever called u= pon > to do > > so, but with a 1G LAN, that's not going to happen very often. I ju= st > wonder > > how many people who complain of poor performance can really benefit= all > that > > much from increased performance? >=20 > 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 w= e 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 t= han 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 o= f our locations is more than 1G. Most are 100M. -- 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