From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Blunck Subject: Re: dm-snapshot scalability - chained delta snapshots approach Date: Tue, 24 Oct 2006 11:52:00 +0200 Message-ID: <20061024095200.GE4729@hasse.suse.de> References: <44DA246B020000B60000BD22@lucius.provo.novell.com> <44DB5BDD020000B60000BD96@lucius.provo.novell.com> <44DB5C21.F25D.00B6.0@novell.com> <451A5915.F25D.00B6.0@novell.com> <20060927091700.GA6314@X40.redhat.com> <62b0912f0610231016r23585c34y6308cb3540ed926c@mail.gmail.com> Reply-To: device-mapper development Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <62b0912f0610231016r23585c34y6308cb3540ed926c@mail.gmail.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: dm-devel-bounces@redhat.com Errors-To: dm-devel-bounces@redhat.com To: dm-devel@redhat.com List-Id: dm-devel.ids On Mon, Oct 23, Molle Bestefich wrote: > >This means that every snapshot still has its own exception store. > >This would make deletion of snapshots unnecessary complex. > > Complex, how? > > Necessary operations (in order listed): > * Acquire exclusive lock on this snapshot. > * Check that next snapshot has room for exceptions, abort if not. > * Acquire exclusive lock on next snapshot. > * Move all exceptions to next snapshot. > * Unlock next snapshot. > * Remove this snapshot. > * Done... > > Sounds simple to me, but maybe I'm missing the point. Hmm, sounds simple. Somehow I can't remember exactly where I thought the problem is ... > >We discussed some of the ideas about snapshots here at the dm summit. The > >general ideas are as follows: > > > >- one exception store per origin device that is shared by all snapshots > > Now that sounds complex. But that is something already implemented for clustered snapshots although that is userspace code. > >Although that includes a complete redesign of the exception store code. > > Especially when you say stuff like that :-). > The chained-snapshots approach needs that too. > >The throughput issues should be addressed by only > >writing to one exception store. > > Wouldn't this make debugging more complex, and further add to > the difficulty of snapshot resizing? Resizing? Nope, you only need to resize the exception store thats it. Resizing the chained-snapshots approach is complex however: in the worst case you have to move the exception stores to get enough free space.