From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx2.redhat.com (mx2.redhat.com [10.255.15.25]) by int-mx2.corp.redhat.com (8.13.1/8.13.1) with ESMTP id l4AEXFvn002172 for ; Thu, 10 May 2007 10:33:16 -0400 Received: from ik-out-1112.google.com (ik-out-1112.google.com [66.249.90.181]) by mx2.redhat.com (8.13.1/8.13.1) with ESMTP id l4AEXBpI012193 for ; Thu, 10 May 2007 10:33:12 -0400 Received: by ik-out-1112.google.com with SMTP id b35so388711ika for ; Thu, 10 May 2007 07:33:11 -0700 (PDT) Message-ID: <87f94c370705100733o2d6fb499q8370f801c6833b42@mail.gmail.com> Date: Thu, 10 May 2007 10:33:11 -0400 From: "Greg Freemyer" Subject: Re: [linux-lvm] Should I expect snapshot origin LV's to be 10x slower? In-Reply-To: <55c223960705100654m36b559cckada97fccb09a42ac@mail.gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <55c223960705100654m36b559cckada97fccb09a42ac@mail.gmail.com> 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"; format="flowed" To: LVM general discussion and development On 5/10/07, Alex Owen wrote: > Hello, > I have just been making some snapshot performance benchmarks on a > Debian Etch system. > Kernel: 2.6.18-4-686 (2.6.18.dfsg.1-12etch1) > dmsetup: 1.02.08-1 > lvm2: 2.02.06-4 > > I have been using commands of the form: > time dd if=/dev/zero of=/dev/volgroup/test bs=1M count=100 > to get speeds for copying to a LVM device both WITH and WITHOUT a > single snapshot. > > It seems that writes take >=10 times longer the first time a newly > snapshot origin device is written to. > > I was expecting somthing like a 2x or 3x performance loss as 1 > physical read and 2 physical writes must occur for a single logical > write. I was NOT expecting there to be a 10x overhead. As I move to > larger devices (bs=1M count=1000) the 10x figure rises to nearer 20x. > This is also true on mounted origin LV's. > > Has anyone else benchmarked this? Is this normal? > > Thanks for any feedback > Alex Owen I always ensure my snapshots are on physically separate drives than my origin. If they are on the same drive I'm not surprised you're having speed issues. You are significantly increasing the amount of disk seek activity. Having in separate drives should be much better. (FYI: It has been a while since I benchmarked, so you may still have problems.) Greg -- Greg Freemyer The Norcross Group Forensics for the 21st Century