From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gabor Gombas Subject: Re: [Aoetools-discuss] [Xen-devel] domU is causing misaligned disk writes Date: Wed, 28 Apr 2010 21:55:29 +0200 Message-ID: <20100428195528.GA10354@twister.home> References: <20100420080958.GN5660@tracyreed.org> <20100420084955.GV1878@reaktio.net> <20100420200004.GQ5660@tracyreed.org> <20100420202519.GB9220@phenom.dumpdata.com> <20100420211913.GV5660@tracyreed.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8BIT Return-path: Content-Disposition: inline In-Reply-To: <20100420211913.GV5660@tracyreed.org> Sender: linux-kernel-owner@vger.kernel.org To: Tracy Reed , Konrad Rzeszutek Wilk , Pasi =?iso-8859-2?Q?K=E4rkk=E4inen?= , xen-devel@lists.xensource.com, Aoetools-discu List-Id: xen-devel@lists.xenproject.org On Tue, Apr 20, 2010 at 02:19:13PM -0700, Tracy Reed wrote: > > How do you know this is a mis-aligned sectors issue? Is this what your > > AOE vendor is telling you ? > > No AoE vendor involved. I am using the free stuff. I think it is a > misalignment issue because during a purely write test it is doing > massive amounts of reading according to iostat. How about actually verifying that by e.g. using wireshark and comparing the I/O patterns in the fast and slow cases? The differences in the patterns may give clues where to look further. > #dd if=/dev/zero of=/dev/etherd/e6.1 oflag=direct bs=4096 > count=3000000 > 1764883+0 records in > 1764883+0 records out > 7228960768 bytes (7.2 GB) copied, 402.852 seconds, 17.9 MB/s > > But even on my local directly attached SATA workstation disk when > doing that same dd on an otherwise idle machine I see performance > like: > > $ dd if=/dev/zero of=foo.test bs=4096 count=4000000 > C755202+0 records in > 755202+0 records out > 3093307392 bytes (3.1 GB) copied, 128.552 s, 24.1 MB/s > > which again suggests that oflag=direct isn't doing quite what I expect. oflag=direct turns off caching on the host dd is running on, i.e. the initiator. The target still caches writes of course, unless you tell it not to by passing the "-d" flag to vblade. Gabor