[Cluster-devel] [Lsf-pc] [LSF/MM ATTEND] [TOPIC] fs/block interface discussions

[Jan Kara <jack@suse.cz>]

  Hi,

On Fri 12-12-14 11:46:34, Steven Whitehouse wrote:
> 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).
> >
> 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.
  I think understand your idea with a 'barrier'. It's just that I have
troubles seeing how it would actually get implemented - how do you make
sure that e.g. after writing back block allocation bitmap and while writing
back other metadata, noone can allocate new blocks to file 'foo' and still
writeback the file's inode before you submit the barrier?

> 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,
  Sure, if you have some proposal (not necessarily patches) then it's
probably worth talking about.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR