From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx3.redhat.com (mx3.redhat.com [172.16.48.32]) by int-mx1.corp.redhat.com (8.13.1/8.13.1) with ESMTP id l9IG2cHD016763 for ; Thu, 18 Oct 2007 12:02:38 -0400 Received: from e2.ny.us.ibm.com (e2.ny.us.ibm.com [32.97.182.142]) by mx3.redhat.com (8.13.1/8.13.1) with ESMTP id l9IG2Vrh024582 for ; Thu, 18 Oct 2007 12:02:31 -0400 Received: from d01relay04.pok.ibm.com (d01relay04.pok.ibm.com [9.56.227.236]) by e2.ny.us.ibm.com (8.13.8/8.13.8) with ESMTP id l9IG2Nc1024857 for ; Thu, 18 Oct 2007 12:02:23 -0400 Received: from d01av04.pok.ibm.com (d01av04.pok.ibm.com [9.56.224.64]) by d01relay04.pok.ibm.com (8.13.8/8.13.8/NCO v8.5) with ESMTP id l9IG2FLl107920 for ; Thu, 18 Oct 2007 12:02:23 -0400 Received: from d01av04.pok.ibm.com (loopback [127.0.0.1]) by d01av04.pok.ibm.com (8.12.11.20060308/8.13.3) with ESMTP id l9IG25fu025913 for ; Thu, 18 Oct 2007 12:02:05 -0400 Date: Thu, 18 Oct 2007 09:01:45 -0700 From: malahal@us.ibm.com Subject: Re: [linux-lvm] very slow fw performance when snapshots active Message-ID: <20071018160145.GA14967@us.ibm.com> References: <1A2D7011B6705B449F66B36B232097C07A7597@x1.ads.i-dmedia.com> <47177953.4020008@oxeva.fr> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <47177953.4020008@oxeva.fr> Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Gabriel Barazer Cc: LVM general discussion and development Reducing chunksize may help a bit but you may end up having performance problems when you read snapshot data later. Thanks, Malahal. Gabriel Barazer [gabriel@oxeva.fr] wrote: > On 10/18/2007 4:01:40 PM +0200, "Zurell, Falko" > wrote: > >Ok, i understand the point with the swap file of vi. But why is this > >with snapshots activated that much slower than without? > >Why can the file be saved in seconds without cow instead of about 2 > >minutes with cow. Needing twice the time for saving would be reasonable. > > I think this is because the re-writing operation on a standard block > device is done using a comparison between the data to write, and the > data already written (which is nearly all the time read from the OS file > cache in your test case, instead of the block device itself, because the > file written was entirely read just before) whereas the LVM snapshot > target redirect any write to the COW space without this comparison step. > Maybe there is a possible optimization of the snapshot dm target which > could compare block before deciding to write them on the COW space. > Obvisouly, such optimization process add a slight overhead an can > possibly have a negative impact on the performance. > > Gabriel > > _______________________________________________ > linux-lvm mailing list > linux-lvm@redhat.com > https://www.redhat.com/mailman/listinfo/linux-lvm > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/