> -----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? -- 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
