From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx1.redhat.com (ext-mx13.extmail.prod.ext.phx2.redhat.com [10.5.110.18]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id p1O7YQHU004492 for ; Thu, 24 Feb 2011 02:34:27 -0500 Received: from abpni.co.uk (host-92-27-106-173.static.as13285.net [92.27.106.173] (may be forged)) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id p1O7YDbm027618 for ; Thu, 24 Feb 2011 02:34:14 -0500 Message-ID: <4D6609E4.10800@abpni.co.uk> Date: Thu, 24 Feb 2011 07:33:56 +0000 From: Jonathan Tripathy MIME-Version: 1.0 References: <4D64FF3C.6080602@abpni.co.uk> <1298466573.19562.147.camel@ubuntu> <4D65124C.1070505@abpni.co.uk> <1298470564.19562.150.camel@ubuntu> <4D651735.1000802@abpni.co.uk> <20110223101259.77143753@bettercgi.com> <4D653BEF.5010600@abpni.co.uk> <4D654FBD.8030504@abpni.co.uk> <4D655459.6050806@gmail.com> <4D656817.6060900@gmail.com> <4D6572C0.6070008@abpni.co.uk> <4D65A1A9.1040205@abpni.co.uk> <4D65A839.50107@abpni.co.uk> <4D65A8F5.8040606@abpni.co.uk> In-Reply-To: Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] Snapshots and disk re-use 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 24/02/11 02:00, Stuart D. Gathman wrote: > On Thu, 24 Feb 2011, Jonathan Tripathy wrote: > >>>>> So I'm guessing then, that when a snapshot is created for an origin, >>>>> then >>>>> there are 2 physical copies of the data on disk? (Albeit only one is >>>>> accessible at the regular filesystem level) >>>> NO, NO, NO. There is still only *one* physical copy of the data >>>> after creating a snapshot. You have simply created a "branch point" >>>> which can now diverge as each branch is written to. >>>> >>> Then why was it suggested that I should zero my new customer LVs upon >>> creation? Please remember that my snapshots will not be written to >>> >> However the origin will be written to as it will be in use... > There were 3 cases of what you might be asking. One of the 3 cases was: > If you are taking snapshots for backup, then it was suggested to zero the *-cow > (which will have any blocks written to the origin since the snapshot was taken) > before deleting the snapshot. However, I wasn't sure if this was safe to while > the origin is mounted, since writes to the origin consult the *-cow to see > whether origin blocks need to be copied before begin overwritten. But I want to keep this data. As the data written to the origin while a respective snapshot is in use would be data during normal operation of a customer VPS. All I want to do is make sure that my use of snapshot for backup purposes (to rsync the snapshot to a remote server) doesn't leave loose data lying anywhere on physical disk.