From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: Awful RAID5 random read performance Date: Mon, 01 Jun 2009 13:01:57 +0100 Message-ID: <4A23C335.30306@anonymous.org.uk> References: <4A21A938.6050907@harddata.com> <4A2222E6.2070705@msgid.tls.msk.ru> <200905310147.58119.tfjellstrom@shaw.ca> <4A227832.1090808@anonymous.org.uk> <874ov1w5po.fsf@frosties.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <874ov1w5po.fsf@frosties.localdomain> Sender: linux-raid-owner@vger.kernel.org To: Linux RAID List-Id: linux-raid.ids On 31/05/2009 18:19, Goswin von Brederlow wrote: > John Robinson writes: >> Clearly when you're doing this over 4 drives you can have ~400 >> seeks/second but that's still limiting you to ~400 reads/second for >> smallish block sizes. > > Note that that only holds true for writes or multithreaded reads. > Reading from a single thread will randomly pick one drive (depending > on where it wants to read), wait for it to seek, read one block of > data and repeat. So you get the speed of a single drive no matter how > many drives there are in the raid. Sure, that's why I said "can", but I thought iozone was multi-threaded. Maybe it needs an option specified, in which case using more threads than there are discs would be a good idea. Cheers, John.