Cross-subvolume rename behavior

[Sean Greenslade <sean@seangreenslade.com>]

Hello, all. I'm currently tracking down the source of some strange
behavior in my setup. I recognize that this isn't strictly a btrfs
issue, but I figured I'd start at the bottom of the stack and work my
way up.

I have a server with a btrfs filesystem on it that I remotely access on
several systems via an sshfs mount. For the most part this works
perfectly, but I just discovered that moving files between subvolumes on
that mount fails with a confusing "Operation not permitted" error.

After doing some digging, it turns out it's not actually a permissions
error. If I do the same operation locally on the server, it succeeds,
but an strace of the mv reveals that the rename() syscall returns EXDEV.
The mv util takes this as a sign to fall back on the copy-and-delete
routine, so the move succeeds. Unfortunately, it seems that somewhere in
sshfs, sftp, or fuse, the EXDEV is getting turned into a generic
failure, which mv apparently interprets as "permission denied".

So my question for the btrfs devs: is rename()-ing across subvolumes
not feasible, or is this simply a case of no one has implemented that
yet? 

Thanks for any insights,

--Sean