Re: [PATCH 1/2] ovl: support freeze/thaw super

["Serge E. Hallyn" <serge@hallyn.com>]

Quoting Amir Goldstein (amir73il@gmail.com):
> On Mon, Jan 23, 2017 at 3:23 PM, Pavel Emelyanov <xemul@virtuozzo.com> wrote:
> > On 01/23/2017 11:47 AM, Amir Goldstein wrote:
> >> On Thu, Jan 19, 2017 at 2:13 PM, Amir Goldstein <amir73il@gmail.com> wrote:
> >>> freeze/thaw of upper is all that is needed.
> >>>
> >>
> >> Miklos,
> >>
> >> Looking at it again, I believe that not even that is needed.
> >> Having fixed ovl_sync_fs() with patch #2, ovl_freeze()
> >> and ovl_unfreeze() need to be NOP. Am I right?
> >>
> >> In fact, freezing upper fs, when many overlayfs mounts
> >> share the same base fs (a-la docker) would be quite lame.
> >>
> >> WRT other use cases of freezing overlayfs, I am not sure
> >> if it is needed for correctness of docker checkpoint/restart?
> >>
> >> Pavel?
> >
> > Thanks for the heads-up :)
> >
> > Good question. Actually we haven't yet experimented with doing _full_
> > (with FS) snapshot of containers, but at the first glance I can't
> > see the need for OVL freeze :( We stop all the processes in entry.S
> > effectively, so no in-flight IO can be happening.
> >
> 
> Serge, Stephan,
> 
> Same question.
> 
> Would lxc-snapshot gain anything from the ability to fsfreeze an overlay
> mount?

lxc-snapshot only works on stopped containers.  'lxc snapshot' can do live
snapshots using criu.  Tycho, does that do anything right now to freeze the
fs?  I'm not sure that freezing all the tasks is necessarily enough to settle
the fs, but I assume you're doing something about that already?

> Basically, it should give you the ability to create a consistent snapshot
> of overlayfs upper dir from a running container, but maybe you already do
> that by freezing the container processes?
> I couldn't figure that out from lxc-snapshot documentation.
> 
> The context is that I implemented overlayfs fsfreeze for my own needs,
> but I need other use cases to justify merging the feature.
> 
> Thanks,
> Amir.