From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: Linux RAID & XFS Question - Multiple levels of concurrency = faster I/O on md/RAID 5? Date: Sat, 01 Nov 2008 12:14:44 +0000 Message-ID: <490C4834.3050404@anonymous.org.uk> References: <490C359F.7080504@anonymous.org.uk> 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: Justin Piszcz Cc: linux-raid@vger.kernel.org, xfs@oss.sgi.com List-Id: linux-raid.ids On 01/11/2008 12:00, Justin Piszcz wrote: > On Sat, 1 Nov 2008, John Robinson wrote: >> On 01/11/2008 08:29, Justin Piszcz wrote: >> [...] >>> Why is running 3 jobs con-currently that take care of two parts each >>> more than >>> twice as fast than running one job for six parts? >> >> Because you have multiple CPUs? > > So 1/4 of a quad core q6600 cannot achieve higher rates of I/O due to the > parity operations being that costly? > > Is the only way to increase the single-threaded speed to increase the > maximum > CPU core speed/get a faster CPU, and/or theoretically a multi-threaded > md-raid > could maximize throughput? Actually I was thinking that your test job - I think you said it used tar - is single-threaded and CPU-bound on one core, and doesn't saturate the MD subsystem. Your jobs are 75% user time to 25% system time, and the user time is not parellelisable until you do it yourself by splitting the work up. Cheers, John.