From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Whitehouse Date: Fri, 12 Dec 2014 11:46:34 +0000 Subject: [Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions In-Reply-To: <20141211005222.GB26014@agk-dp.fab.redhat.com> References: <5488335C.9090605@redhat.com> <20141210124833.GG25671@quack.suse.cz> <548854EE.1020901@redhat.com> <20141210184651.GB9398@quack.suse.cz> <20141211005222.GB26014@agk-dp.fab.redhat.com> Message-ID: <548AD59A.8000007@redhat.com> List-Id: To: cluster-devel.redhat.com MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Hi, On 11/12/14 00:52, Alasdair G Kergon wrote: > On Wed, Dec 10, 2014 at 07:46:51PM +0100, Jan Kara wrote: >> But still you first need to stop all writes to the filesystem, then do a >> sync, and then allow writing again - which is exactly what freeze does. > And with device-mapper, we were asked to support the taking of snapshots > of multiple volumes simultaneously (e.g. where the application data is > stored across more than one filesystem). Thin dm snapshots can handle > this (the original non-thin ones can't). > > Alasdair > Thats good to know, and a useful feature. One of the issues I can see is that because there are a number of different layers involved (application/fs/storage) coordination of requirements between those is not easy. To try to answer Jan's question earlier in the thread, no I don't know any specific application developers, but I can certainly help to propose some kind of solution, and then get some feedback. I think it is probably going to be easier to start with a specific proposal, albeit tentative, and then ask for feedback than to just say "how should we do this?" which is a lot more open ended. Going back to the other point above regarding freeze, is it not necessarily a requirement to stop all writes in order to do a snapshot, what is needed is in effect a barrier between operations which should be represented in the snapshot and those which should not, because they happen "after" the snapshot has been taken. Not that I'm particularly attached to that proposal as it stands, but I hope it demonstrates the kind of thing I had in mind for discussion. I hope also that it will be possible to come up with a better solution during and/or following the discussion. The goal would really be to figure out which bits we already have, which bits are missing, where the problems are, what can be done better, and so forth, while we have at least two of the three layers represented and in the same room. This is very much something for the long term rather than a quick discussion followed by a few patches kind of thing, I think, Steve.