From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx1.redhat.com (ext-mx11.extmail.prod.ext.phx2.redhat.com [10.5.110.16]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id t0ECcka6021272 for ; Wed, 14 Jan 2015 07:38:46 -0500 Received: from mail.shockmedia.nl (mail.shockmedia.nl [31.7.4.4]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id t0ECciLL010974 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-SHA bits=128 verify=NO) for ; Wed, 14 Jan 2015 07:38:45 -0500 Received: from office.shockmedia.nl ([31.7.3.253] helo=[192.168.8.179]) by mail.shockmedia.nl with esmtpa (Exim 4.82) (envelope-from ) id 1YBNDf-0003mY-9f for linux-lvm@redhat.com; Wed, 14 Jan 2015 13:38:43 +0100 Message-ID: <54B66352.4010902@shockmedia.nl> Date: Wed, 14 Jan 2015 13:38:42 +0100 From: Bram Klein Gunnewiek MIME-Version: 1.0 References: <54B619AE.6020704@shockmedia.nl> <54B62A0C.9080800@redhat.com> <54B64262.2030509@shockmedia.nl> <54B65611.1090407@gmail.com> In-Reply-To: <54B65611.1090407@gmail.com> Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] determining snapshot overhead 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 01/14/2015 12:42 PM, Zdenek Kabelac wrote: > Dne 14.1.2015 v 11:18 Bram Klein Gunnewiek napsal(a): >> Thanks for your reply. I tested it out and it does not seem to work >> (lvm2 >> version used is 2.02.98-6ubuntu2): >> > > yep - too old for this feature being supported. Ok, then we have to do it ourself since we don't want to go with backports / custom versions on our systems > >> lvcreate -n testvolume3 -L 1G vg >> lvcreate -n testvolume3_snapshot -s /dev/vg/testvolume3 -l100%ORIGIN >> dd if=/dev/zero of=/dev/vg/testvolume3 bs=1G count=1 >> >> After this the snapshot is invalid: >> >> testvolume3_snapshot vg swi-I-s-- 1.00g testvolume3 100.00 >> >> I checked out the lvm2 source code and found the methods >> cow_max_extents and >> _cow_max_size in snapshot_manip.c. Do these methods calculate the >> exact max >> size of the snapshot meta data? > > yes. > > It's probably worth to note there has been some related kernel fixes > for old snaps - otherwise there has been theoretical path to create > much bigger snapshots then the origin size was - though it's nearly > impossible to reach such state. Thanks again. I did some tests and calculations and it seems that the overhead is ~0.39% of the origin size when chunk sizes of 4KiB are used. Bigger chunk size means less overhead. If we use 0.5% of the origin space as meta data overhead we should be on the safe side I think? > Zdenek > > >> >> On 01/14/2015 09:34 AM, Zdenek Kabelac wrote: >>> Dne 14.1.2015 v 08:24 Bram Klein Gunnewiek napsal(a): >>>> Hello, >>>> >>>> I am trying to create snapshots of thick volumes that can't break >>>> because they >>>> overflow (thin volumes are not an option right now). I can't find >>>> information >>>> on how to calculate (in advance) the metadata overhead of a >>>> snapshot when all >>>> blocks in the origin volume are changed (e.g. the snapshot is 100% >>>> full). Is >>>> there a way to do this? >>> >>> >>> Recent versions of lvm2 (let's say >= 111) are able to determine >>> this size >>> automatically - just use: 'lvcreate -s -l100%ORIGIN vg/origin' >>> >>> Regards >>> >>> Zdenek >>> >>> _______________________________________________ >>> 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/ >> >> _______________________________________________ >> 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/ >> > > _______________________________________________ > 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/