Re: [RFC] Btrfs "sendshots" and hidden snapshots

[Chris Mason <chris.mason@fusionio.com>]

On Fri, Jul 06, 2012 at 02:51:43AM -0600, Alexander Block wrote:
> On Fri, Jul 6, 2012 at 12:34 AM, Goffredo Baroncelli <kreijack@libero.it> wrote:
> > On 07/05/2012 06:51 PM, Alexander Block wrote:
> >> Hello all,
> >>
> >> in IRC we had a discussion on how we could solve sending live
> >> subvolumes and how to send subvolumes without the need to
> >> administrate/keep old snapshots for incremental sends. One of the
> >> ideas was to introduce "sendshots", which are basically snapshots
> >> where no refs are counted for file data. This means, that when file
> >> data is changed in the sendshot origin, we do not consume extra space
> >> for two copies of the data. We would only have the metadata
> >> duplicated.
> >>
> >> For the initial btrfs send we could do this:
> >> 1. Create a hidden read-only snapshot of the subvolume to send. Hidden
> >> means that it's not referenced by any subvolume. It is however still a
> >> normal snapshot (not a sendshot!). Hidden snapshots are not possible
> >> atm so we would have to implement that. This step allows us to send
> >> read-write subvolumes, because we have a freezed version of it.
> >
> > Why we should want/need an hidden snapshot ? We could put this kind of
> > hidden snapshot under a directory dot-prefixed (like /.hidden-subvolumes)
> That would have the problem that the user may modify the subvolume
> in-between (by removing the ro flag). Or he could simple cd into it
> and we would later fail to delete it.

I prefer to make this more explicit.  We could add a hard-readonly flag
that cannot be cleared.  Having the snapshot show in the FS lets the
admin know what things are really using space.

-chris