On Fri, Dec 03, 2010 at 04:45:27PM -0500, Josef Bacik wrote:
> So now that I've actually looked at everything, it looks like the semantics
> are
> all right for subvolumes
>
> 1) readdir - we return the root id in d_ino, which is unique across the fs
Though Michael Vrable pointed out an apparent collision with "normal"
inode numbers on the parent filesystem?
> 2) stat - we return 256 for all subvolumes, because that is their inode number
> 3) dev_t - we setup an anon super for all volumes, so they all get their own
> dev_t, which is set properly for all of their children, see below
>
> [r...@test1244 btrfs-test]# stat .
> File: `.'
> Size: 20 Blocks: 8 IO Block: 4096 directory
> Device: 15h/21d Inode: 256 Links: 1
> Access: (0555/dr-xr-xr-x) Uid: ( 0/ root) Gid: ( 0/ root)
> Access: 2010-12-03 15:35:41.931679393 -0500
> Modify: 2010-12-03 15:35:20.405679493 -0500
> Change: 2010-12-03 15:35:20.405679493 -0500
>
> [r...@test1244 btrfs-test]# stat foo
> File: `foo'
> Size: 12 Blocks: 0 IO Block: 4096 directory
> Device: 19h/25d Inode: 256 Links: 1
> Access: (0700/drwx------) Uid: ( 0/ root) Gid: ( 0/ root)
> Access: 2010-12-03 15:35:17.501679393 -0500
> Modify: 2010-12-03 15:35:59.150680051 -0500
> Change: 2010-12-03 15:35:59.150680051 -0500
>
> [r...@test1244 btrfs-test]# stat foo/foobar
> File: `foo/foobar'
> Size: 0 Blocks: 0 IO Block: 4096 regular empty file
> Device: 19h/25d Inode: 257 Links: 1
> Access: (0644/-rw-r--r--) Uid: ( 0/ root) Gid: ( 0/ root)
> Access: 2010-12-03 15:35:59.150680051 -0500
> Modify: 2010-12-03 15:35:59.150680051 -0500
> Change: 2010-12-03 15:35:59.150680051 -0500
>
> So as far as the user is concerned, everything should come out right.
> Obviously
> we had to do the NFS trickery still because as far as VFS is concerned the
> subvolumes are all on the same mount. So the question is this (and really
> this
> is directed at Christoph and Bruce and anybody else who may care), is this
> good
> enough, or do we want to have a seperate vfsmount for each subvolume? Thanks,
For nfsd's purposes, we need to be able find out about filesystems in
two different ways:
1. Lookup by filehandle: we need to be able to identify which
subvolume we're dealing with from a filehandle.
2. Lookup by path: we need to notice when we cross into a
subvolume.
Looks like #1 already works. Not #2: the current nfsd code just checks
for mountpoints. We could modify nfsd to also check whether dev_t
changed each time it did a lookup. I suppose it would work, though it's
annoying to have to do it just for the case of btrfs.
As far as I can tell, crossing into a subvolume is like crossing a
mountpoint in every way except for the lack of a separate vfsmount. I'd
worry that the inconsistency will end up requiring more special cases
down the road, but I don't have any in mind.
--b.
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