From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: mdadm raid1 read performance Date: Wed, 04 May 2011 01:57:28 +0100 Message-ID: <4DC0A478.9010704@anonymous.org.uk> References: 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: Linux RAID List-Id: linux-raid.ids On 04/05/2011 01:07, Liam Kurmos wrote: > Hi, > > I've been testing mdadm (great piece of software btw) however all my > test show that reading from raid1 is only the same speed as reading > from a single drive. > > Is this a known issue? or is there something seriously wrong with my > system? i have tried v2.8.1 and v.3.2.1 without difference and several > benchmarking methods. This is a FAQ. Yes, this is known. No, it's not an issue, it's by design - pretty much any RAID 1 implementation will be the same because of the nature of spinning discs. md RAID 1 will serve multiple simultaneous reads from the different mirrors, giving a higher total throughput, but a single-threaded read will read from only one. If you want RAID 0 sequential speed at the same time as RAID 1 mirroring, look at md RAID 10, and in particular RAID 10,f2; please see the excellent documentation and wiki for more details. Cheers, John.