From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <51DB0CF6.7070400@redhat.com> Date: Mon, 08 Jul 2013 21:03:18 +0200 From: Zdenek Kabelac MIME-Version: 1.0 References: <731d217f00b6d8f6c16a75cbe167baed.squirrel@mail.mohawksoft.com> <51DAF9AF.900@redhat.com> In-Reply-To: Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] Snapshots of snapshots are not supported yet 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 Cc: markw@mohawksoft.com Dne 8.7.2013 19:57, markw@mohawksoft.com napsal(a): >> Dne 8.7.2013 18:05, markw@mohawksoft.com napsal(a): >>> I'm trying to get a snapshot of a snapshot. I know the new thin >>> provisioning stuff supports this, but I'm not sure the release state is >>> something I can use at a customer. >>> >>> Using LVM2, the snapshot format is tried and true, but inefficient if >>> you >>> want to take multiple snapshots. >>> >>> Looking at dmsetup, there doesn't seem like there would be an issue >>> taking >>> a snapshot of a snapshot, but it is disabled in the tools. >>> >>> I guess my question is this: Can it be done? Is there any reason why it >>> should not work? >>> >> >> 1. Noone has written support for it in lvm2. And AFAICK noone is working >> on this. > > I thought I saw a patch for this in the 2010 time frame. > I'm not aware of any such support >> >> 2. Since the activation order of old snapshot is not as simple as with >> other >> targets, it's currently not so easy to add support for into lvm deptree >> resolver. > > In my case I would, more or less, be maintaining a lot of the > interdependency information for the chain of shapshots and it would > managed by a daemon being controlled by an application, not really subject > to the whims of a sys admin. Beware the old snaps are meant to be used as a 'temporary objects' not something you want to keep for a long time - it's degrading write performance of origin device and there is also no safe resize support - i.e. when you overfill snapshot - it's invalidated. If you would have used such snapshot as a base origin for a chain with not enough space - then whole chain is lost. > Is there a stable and supported way of getting this functionality? FYI: > I'm trying to use RHEL as the basis for this and I think that's an old > 2.6.x kernel. Kernel RHEL6 2.6 is a heavily modified version - so it has a lot of features of 3.X kernel and dm support is basically back ported to very latest version - so you have there thin provisioning support. If you want to use snaps of snaps - I'd really recommend to use thins and report problems rather there - then trying to old snaps. They were not designed for this use-case. Zdenek