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 l4ADshoq029813 for ; Thu, 10 May 2007 09:54:43 -0400 Received: from an-out-0708.google.com (an-out-0708.google.com [209.85.132.242]) by mx2.redhat.com (8.13.1/8.13.1) with ESMTP id l4ADsMTc002742 for ; Thu, 10 May 2007 09:54:42 -0400 Received: by an-out-0708.google.com with SMTP id b33so149028ana for ; Thu, 10 May 2007 06:54:42 -0700 (PDT) Message-ID: <55c223960705100654m36b559cckada97fccb09a42ac@mail.gmail.com> Date: Thu, 10 May 2007 14:54:39 +0100 From: "Alex Owen" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Disposition: inline Subject: [linux-lvm] Should I expect snapshot origin LV's to be 10x slower? 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: linux-lvm@redhat.com 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