From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: Awful RAID5 random read performance Date: Wed, 03 Jun 2009 20:57:34 +0100 Message-ID: <4A26D5AE.2000003@anonymous.org.uk> References: <20090531154159405.TTOI3923@cdptpa-omta04.mail.rr.com> <200905311056.30521.tfjellstrom@shaw.ca> <4A25754F.5030107@tmr.com> <20090602194704.GA30639@rap.rap.dk> <4A25B201.2000705@anonymous.org.uk> <4A26C313.6080700@tmr.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <4A26C313.6080700@tmr.com> Sender: linux-raid-owner@vger.kernel.org To: Linux RAID List-Id: linux-raid.ids On 03/06/2009 19:38, Bill Davidsen wrote: > John Robinson wrote: >> On 02/06/2009 20:47, Keld J=F8rn Simonsen wrote: [...] >>> In your case, using 3 disks, raid5 should give about 210 % of the=20 >>> nominal >>> single disk speed for big file reads, and maybe 180 % for big file >>> writes. raid10,f2 should give about 290 % for big file reads and 14= 0% >>> for big file writes. Random reads should be about the same for raid= 5 and >>> raid10,f2 - raid10,f2 maybe 15 % faster, while random writes should= be >>> mediocre for raid5, and good for raid10,f2. >> >> I'd be interested in reading about where you got these figures from=20 >> and/or the rationale behind them; I'd have guessed differently... >=20 > For small values of N, 10,f2 generally comes quite close to N*Sr, whe= re=20 > N is # of disks and Sr is single drive read speed. This is assuming=20 > fiarly large reads and adequate stripe buffer space. Obviously for=20 > larger values of N that saturates something else in the system, like = the=20 > bus, before N gets too large. I don't generally see more than (N/2-1)= *Sw=20 > for write, at least for large writes. I came up with those numbers ba= sed=20 > on testing 3-4-5 drive arrays which do large file transfers. If you w= ant=20 > to read more than large file speed into them, feel free. Actually it was the RAID-5 figures I'd have guessed differently. I'd=20 expect ~290% (rather than 210%) for big 3-disc RAID-5 reads, and ~140%=20 (rather than "mediocre") for random small writes. But of course I=20 haven't tested. Cheers, John. -- 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