Re: [PATCH 0/2] btrfs: allow cross-subvolume BTRFS_IOC_CLONE

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

Excerpts from Christoph Hellwig's message of 2011-04-01 09:34:05 -0400:
> On Thu, Mar 31, 2011 at 08:02:22AM -0400, Chris Mason wrote:
> > Excerpts from Christoph Hellwig's message of 2011-03-31 02:36:36 -0400:
> > > On Thu, Mar 31, 2011 at 12:00:11AM -0400, Larry D'Anna wrote:
> > > > This is a simple patch to allow reflinks to be made crossing subvolume
> > > > boundaries.
> > > 
> > > NAK.  subvolumes will have to become vfsmounts sooner or later, and we
> > > really must not support any operations spanning mountpoints.
> > > 
> > 
> > Sorry, I disagree here.  reflinks were always intended to be able to
> > span subvolumes.  There's no conflict at all because they span different
> > inodes.
> 
> I don't think it's a good idea to introduce any user visible operations
> over subvolume boundaries.  Currently we don't have any operations over
> mount boundaries, which is pretty fumdamental to the unix filesystem
> semantics.  If you want to change this please come up with a clear
> description of the semantics and post it to linux-fsdevel for
> discussion.  That of course requires a clear description of the
> btrfs subvolumes, which is still completely missing.
> 

The subvolume is just a directory tree that can be snapshotted, and has
it's own private inode number space. 

reflink across subvolumes is no different from copying a file from one
subvolume to another at the VFS level.  The src and destination are
different files and different inodes, they just happen to share data
extents.

-chris