On Fri, May 06, 2011 at 04:20:39PM -0500, Leslie Rhorer wrote: > > -----Original Message----- > > From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid- > > owner@xxxxxxxxxxxxxxx] 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@xxxxxxxxxxxxxxx > > 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@xxxxxxxxx> > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
