btrfs subvolume confusion

[Theo Markettos]

Does anyone here use btrfs? I'm trying to set up a fresh Ubuntu 12.04 beta
machine. The installer has given me this:

atm26local@rama:~$ sudo btrfs subvolume list /
ID 256 top level 5 path @
ID 257 top level 5 path @home

atm26local@rama:~$ mount
/dev/sda1 on / type btrfs (rw,subvol=@)
/dev/sda1 on /home-local type btrfs (rw,subvol=@home)

I want to add another subvolume /local/scratch. So I did
$ sudo mkdir /local
$ cd /local
$ sudo btrfs subvolume create scratch

and I now have:
atm26local@rama:~$ sudo btrfs subvolume list /
ID 256 top level 5 path @ (this is the root fs)
ID 257 top level 5 path @home
ID 258 top level 5 path @/local/scratch

and I appear to have a directory /local/scratch I can create files in. But
it's not listed in the 'mount' table. It still exists if I reboot. If I
try to mount it:
$ mkdir /tmp/foo
$ sudo mount -o subvol=@/local/scratch /dev/sda1 /tmp/foo

That gives me the same contents as /local/scratch. But if I do:
$ mkdir /tmp/bar
$ sudo mount /dev/sda1 /tmp/bar
$ ls /tmp/bar
@ @home

In other words it's somehow different from the system-created subvolumes.

Is it simply the fact that the name has slashes in it? How do I specify a
different name? Do I need to mount the subvolume manually (like @home is
mounted) or can I rely on it being a child of the / subvolume (does this
have implications for snapshotting - if I snapshot / do I also snapshot
/local/scratch too? That might be undesirable). How do I unmount it? For
example, Ubuntu originally mounted @home at /home, which I changed to
/home-local in fstab. There remained nothing at /home (good, because I want
to put a network share there). But it looks like /local/scratch is part of
the / filesystem and can't be unmounted.

I'm very confused...

Thanks
Theo