[linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging

[Chris Cox]

On Tue, 2008-06-03 at 19:38 -0400, Brian J. Murrell wrote:
> On Tue, 2008-06-03 at 15:43 -0500, Chris Cox wrote:
> > 
> > I would think this would be VERY hard to do since both the "origin" and
> > "snapshot" change (file deletes, etc).
> 
> I don't think it's as difficult as you are thinking it is.  You are
> thinking of synchronizing filesystems, at the filesystem level, but
> merging a snapshot back up into the origin does not deal with files or
> even filesystems.  It deals with block devices and blocks, and is really
> nothing more than a coordinated:
> 
> for each COW_block in snapshot; do
>     for other_snapshot of origin; do
>         copy origin[COW_block] to other_snapshot[COW_block]
>         copy snapshot[COW_block] to origin[COW_block]
>     done
> done
> 
> > Do you have a set of rules you
> > use for the merge?
> 
> There is no filesystem level merge.  There are no rules needed.

Ok... I'm still a bit confused (sorry)... I'm going to assume
a last out wins scenario with regards to meta data conflicts
... yes.. I know, I'm having a tough time separating out the
filesystem element from the block device :)

Thanks for the answer... it does make sense (just makes my
brain hurt).