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

[Goffredo Baroncelli <kreijack@libero.it>]

On 07/06/2012 01:55 PM, Chris Mason wrote:
> 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.


Me too, but I am guessing what should happens when the users try to read
an old data ? (I am talking about sendshot ). If I understood correctly
the old data isn't tracked by the sendshot.

GB