From mboxrd@z Thu Jan 1 00:00:00 1970 From: Brad Campbell Subject: Re: mdadm raid1 read performance Date: Thu, 05 May 2011 07:36:13 +0800 Message-ID: <4DC1E2ED.6070201@fnarfbargle.com> References: <20110504105822.21e23bc3@notabene.brown> <4DC0F2B6.9050708@fnarfbargle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Sender: linux-raid-owner@vger.kernel.org To: Liam Kurmos Cc: Roberto Spadim , Brad Campbell , Drew , NeilBrown , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 05/05/11 07:08, Liam Kurmos wrote: > Thanks to all who replied on this. > > I somewhat naively assumed that having 2 disks with the same data > would mean a similar read speed to raid0 should be the norm (and i > think this is a very popular miss-conception). > I was neglecting the seek time to skip alternate blocks which i guess > must the flaw. > > In theory though if i was reading a larger file, couldn't one disk > start reading at the beginning to a buffer and one start reading from > half way ( assuming 2 disks) and hence get close to 2x single disk > speed? > > as a separate question, what should be the theoretical performance of raid5? > > in my tests i read 1GB and throw away the data. > dd if=/dev/md0 of=/dev/null bs=1M count=1000 > > With 4 fairly fast hdd's i get > > raid0: ~540MB/s > raid10: 220MB/s > raid5: ~165MB/s > raid1: ~140MB/s (single disk speed) > > for 4 disks raid0 seems like suicide, but for my system drive the > speed advantage is so great im tempted to try it anyway and try and > use rsync to keep constant back up. > Try RAID10 with the far layout. It should give you streaming reads the same as RAID0 Brad