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 mABG5ZM0021585 for ; Tue, 11 Nov 2008 11:05:35 -0500 Received: from ciao.gmane.org (main.gmane.org [80.91.229.2]) by mx3.redhat.com (8.13.8/8.13.8) with ESMTP id mABG5I3I001063 for ; Tue, 11 Nov 2008 11:05:19 -0500 Received: from list by ciao.gmane.org with local (Exim 4.43) id 1Kzvju-0000yn-Q9 for linux-lvm@redhat.com; Tue, 11 Nov 2008 16:05:14 +0000 Received: from nu246.asfh-berlin.de ([192.124.246.246]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Tue, 11 Nov 2008 16:05:14 +0000 Received: from gator_ml by nu246.asfh-berlin.de with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Tue, 11 Nov 2008 16:05:14 +0000 From: Peter Daum Date: Tue, 11 Nov 2008 17:04:54 +0100 Message-ID: References: <1c748a490811100711m7b09f422nb1f4eeced5d2e2bb@mail.gmail.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit In-Reply-To: <1c748a490811100711m7b09f422nb1f4eeced5d2e2bb@mail.gmail.com> Sender: news Subject: [linux-lvm] Re: write performance with active snapshot Reply-To: gator_ml@yahoo.de, 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"; format="flowed" To: linux-lvm@redhat.com Hi, Larry Dickson wrote: > My guess is that you are getting the typical seek overhead. Have you > tried making a volume group out of two separate RAID arrays (driving > different spindles), and using lvdisplay --maps to make sure the parent > volume is on one array, the snapshot(s) on the other? That was my suspicion, too (although I could not imagine such an extreme impact). Just for testing I added a single disk to the same volume group and put the snapshot onto that disk - amazingly it made hardly any difference (Actually, I'm almost glad about that, because the combination of a 12-disk-array with a single disk would be under almost all other aspects foolish). One thing that does improve the performance a little (actually by 100%, which in this case meens still pretty lousy 16 MB/sec) is to increase the chunk size to 512kb. (I don't know yet, how this might affect performance when dealing with many small files) ... Regards, Peter > On 11/9/08, *Peter Daum* > > wrote: > > Hi, > > for an application I am just working on it looks like lvm snapshots > would > be just what I need as far as functionality is concerned. Unfortunately, > I am experiencing such a massive degradation in performance, that the > result is almost useless. > > I'm working on a fairly fast machine (Quadcore, 8GB RAM) with a big > hardware RAID array and lvm2 (Debian Lenny; Linux 2.6.26-1-amd64; > LVM version:2.02.39 (2008-06-27) > Library version: 1.02.27 (2008-06-25) > Driver version: 4.13.0) > > Sequentially writing to a file (ext3) on a logical volume, I get a > sustained performance of ~ 250 MB/sec. When I create a snapshot > volume, the write throughput drops to 7-8 MB/secs (on the original > volume; writing to the snapshot I see a significant degradation, > but not nearly, as bad; read performance is o.k.).Is this "normal" > or is there anything I can do to about it? > > I looked in this list and searched the WWW but couldn't find any > concrete information on the performance impact of snapshots > (except http://www.nikhef.nl/~dennisvd/lvmcrap.html). > It seems like write performance should probably be less then 1/3 > of the original throughput, because every write to the source > volume causes 3 I/O operations plus some overhead for meta data. > More difficult to estimate would be the time lost by additional > head movements. Still, a throughput degradation by a factor of 30 > seems pretty extreme. > > Any ideas? > > Regards, > Peter Daum