From mboxrd@z Thu Jan 1 00:00:00 1970 From: Konrad Rzeszutek Wilk Subject: Re: bad write performance with qdisk with larger files in pv-domU Date: Fri, 3 Jun 2011 19:16:39 -0400 Message-ID: <20110603231639.GA21830@dumpdata.com> References: <201106032349.47095.ronny.hegewald@online.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <201106032349.47095.ronny.hegewald@online.de> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Ronny Hegewald Cc: xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org On Fri, Jun 03, 2011 at 11:49:47PM +0000, Ronny Hegewald wrote: > Im using the following 32-bit setup: > > - xen 4.1.0 > - upstream linux-kernel 2.6.39 as dom0 > - linux 2.6.32 pv-domU that has several ext3 partitions mounted with qdisk > (same behaviour with a 2.6.39 kernel, so i continued the investigation with > the 2.6.32er kernel) > > Die read performance is good (ca. 60 MB/s) > > For smaller files (< 30-40 MB) the write-speed is ok. > > But if i copy a larger file (ca > 40 MB), the write speed decreases to ca. 0,5 > MB/s, after the first ca. 40 MBs are written. > > One reason for the bad performance might be that qdisk doesnt use AIO. For > testing purposes i activated AIO in hw/xen_disk.c (i set use_aio=1), but the > domU freezed shortly after the domU-kernel started. You could also use this patch: http://darnok.org/xen/qdisk_vs_blkback_v3.1/qemu-enable-aio.patch But why not use the 3.0-rc1 with the xen-blkback? Or if you want to use 2.6.39 you could use the git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git stable/2.6.39.x tree > > Is this performance-impact expected when no AIO is used? Yeah, it is slow. > > I compared the raw-block implementation from xen-qemu 4.1.0 and current > upstream, in case xen-qemu has some missing bugfixes and found the following > patch that looks a bit interesting > > commit 4899d10d142e97eea8f64141a3507b2ee1a64f52 > Author: Stefan Hajnoczi > Date: Mon Apr 19 13:34:11 2010 +0100 > raw-posix: Use pread/pwrite instead of lseek+read/write > > This patch combines the lseek+read/write calls to use pread/pwrite > instead. This will result in fewer system calls and is already used by > AIO. > > > From the first look the patch cannot be backported 1:1, so i havent tried it > yet, because i doubt that it can make such a huge difference. Or would it be > worth a try? > > Any other ideas how/what to investigate this issue further, in case the write- > speed should be better also without AIO? I know that the qdisk implementation > is expected to be slower, but i would expect at least lets say 5 MB/s. > > > > > _______________________________________________ > Xen-devel mailing list > Xen-devel@lists.xensource.com > http://lists.xensource.com/xen-devel