Re: [PATCH] btrfs-progs: add options to sync filesystem after subvol delete

[Chris Mason <clm@fb.com>]

Quoting David Sterba (2013-12-10 12:36:28)
> On Tue, Dec 10, 2013 at 08:17:07AM -0500, Chris Mason wrote:
> > Quoting David Sterba (2013-12-09 18:32:45)
> > > On Mon, Dec 02, 2013 at 05:02:49PM +0800, Wang Shilong wrote:
> > > > >So an enahced interface could look like this:
> > > > >
> > > > >subvol delete:
> > > > >--commit-each - run the ioc sync/wait ioctl after each delete ioctl
> > > > >--commit-after - dtto but sync/wait after all are deleted
> > > > >--wait-for-cleanup - wait until all given subvols are cleaned
> > > > >
> > > > >'filesystem sync' exteded to wait for subvol cleanup has following
> > > > >cases:
> > > > >- wait for a specific subvolume to be cleaned
> > 
> > It may be hard to wait for a specific subvolume from btrfs fi sync.
> > You'd have to know the id, or have an interface that shows a list of ids
> > currently under deletion (not a bad idea ;)
> 
> It is there, 'btrfs subvol list -d /path', but I'd rather not let
> everybody parse output the output for a simple check.

Aha ;) Yeah, that's what I was thinking.  It's possible to find but hard
enough that nobody would be happy using it.

> 
> > > > >- wait for all currently deleted, do not care if more subvols are
> > > > >   deleted in the meantime
> > > > >- wait until there are no subvolumes left to clean
> > > > I think it is unnecessary to add such options for 'filesystem sync'.
> > > > we may wait a long time until all subvolume deletion are finished as
> > > > async subvolume deletion is implemented in cleaner thread.:-)
> > > 
> > > I mean that 'filesystem sync' will stay as it is now, but will be
> > > enhanced with a few options to further specify what else should be
> > > synced.
> > 
> > It's more natural to have the waiting in the subvol delete command, but
> > I'm not against adding a few ways to wait in btrfs fi sync too, as long
> > as they share the same core implementation.
> 
> Yeah it's natural and I guess it'll be the most frequent type of use.
> I've tried to think of the possible uses so we don't miss anything
> during design phase.
> 
> Knowing the ID is necessary, the subvolume path is lost at deletion
> time, but let's say that the ID is available somehow, eg from
> application logs.
> 
> I have a prototype for that, there's a separate subcommand to do just
> the waiting for a given subvolume list id to be cleaned up (but could
> be merged to fi sync if desired). It's built around the SEARCH ioctl and
> does not need kernel support.

So the part where I think we agree is I don't think the kernel subvol
deletion ioctl should do the waiting.  Exactly how the progs wait is a
different question.

-chris