From mboxrd@z Thu Jan 1 00:00:00 1970 From: Bill Davidsen Subject: Re: Mustn't be RAID 1 and 0 read-performance be similar? Date: Thu, 09 Aug 2007 17:37:36 -0400 Message-ID: <46BB8920.9090105@tmr.com> References: <735D322AED9CD711BE53000476917CD00281A96B@NT-SERVER-5> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <735D322AED9CD711BE53000476917CD00281A96B@NT-SERVER-5> Sender: linux-raid-owner@vger.kernel.org To: "Rustedt, Florian" Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Rustedt, Florian wrote: > Hello List, > > If the speed on RAID 0 is based on reading out in parallel, then it must be > the same on RAID 1, mustn't it? > > On RAID 1, it is possible, to read two blocks in parallel to speed up, too. > > I tried to measure this some weeks ago, but i couldn't get over the > read-performance of a single disk on my raid 1, so that means, that the > software-raid does not use this easy possibility to speed up? > We've had this discussion before, and the RAID code does not take advantage of running reads in parallel to satisfy readahead or anything else. I seen it said that this is on a per-thread basis, but a simple test suggests that it's on a process basis, setting two threads to read alternating 10k blocks as fast as possible ran no faster than a single thread. However, running against RAID-10, even a single thread seemed to run far faster than the speed of a single drive. Lightly tested, don't take this as the last word. -- bill davidsen CTO TMR Associates, Inc Doing interesting things with small computers since 1979