Fwd: What to do about subvolumes?

["Paweł Brodacki" <pawel.brodacki@googlemail.com>]

2010/12/2 David Pottage <david@electric-spoon.com>:
>
> Therefore I think it is a bad idea if potentially different files on =
btrfs
> can have the same inode number. It will break all sorts of tools.
>
> Instead of maintaining a big complicated reference count of used inod=
e
> numbers, could btrfs use bit masks to create a the userland visible i=
node
> number from the subvolume id and the real internal inode number. Some=
thing
> like:
>
> userland_inode =3D ( volume_id << 48 ) & internal_inode;
>
> Please forgive me if this is impossible, or if that C snippet is
> syntactically incorrect. I am not a filesystem or kernel developer, a=
nd I
> have not coded in C for many years.
>
> --
> David Pottage
>

Expanding on the idea: what about a pool of IDs for subvolumes and
inode numbers inside a subvolume having the subvolume ID as a prefix?
It gives each inode a unique number, doesn't require cheating the
userland and is less costly than keeping reference count for each
inode. The obvious downside that I can see is limitation on number of
subvolumes that it would be possible to create. It also lowers the
maximum number of inodes in a filesystem (because of bits taken up by
subvolume ID). I expect there are also less-than obvious downsides.

Just an idea by a kernel and FS ignorant.

--
Pawe=C5=82 Brodacki
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