Re: strange btrfs sub list output

[Andreas Philipp <philipp.andreas@gmail.com>]

On 27.05.2011 10:01, Stephane Chazelas wrote:
> 2011-05-26 22:22:03 +0100, Stephane Chazelas: [...]
>> I get a btrfs sub list output that I don't understand:
>>
>> # btrfs sub list /backup/ ID 257 top level 5 path
>> u1/linux/lvm+btrfs/storage/data/data ID 260 top level 5 path
>> u2/linux/lvm/linux/var/data ID 262 top level 5 path
>> u1/linux/lvm+btrfs/storage/data/snapshots/2010-10-11 ID 263 top
>> level 5 path u2/linux/lvm/linux/home/snapshots/2011-04-07 ID 264
>> top level 5 path u2/linux/lvm/linux/root/snapshots/2011-04-07 ID
>> 265 top level 5 path u2/linux/lvm/linux/var/snapshots/2011-04-07
>> ID 266 top level 5 path
>> u1/linux/lvm+btrfs/storage/data/snapshots/2010-10-26 ID 267 top
>> level 5 path u1/linux/lvm+btrfs/storage/data/snapshots/2010-11-08
>> ID 268 top level 5 path
>> u1/linux/lvm+btrfs/storage/data/snapshots/2010-11-22 ID 269 top
>> level 5 path u1/linux/lvm+btrfs/storage/data/snapshots/2010-12-15
>> ID 270 top level 5 path
>> u2/linux/lvm/linux/home/snapshots/2011-04-14 ID 271 top level 5
>> path u2/linux/lvm/linux/root/snapshots/2011-04-14 ID 272 top
>> level 5 path u2/linux/lvm/linux/var/snapshots/2011-04-14 ID 273
>> top level 5 path
>> u1/linux/lvm+btrfs/storage/data/snapshots/2010-12-29 ID 274 top
>> level 5 path u1/linux/lvm+btrfs/storage/data/snapshots/2011-01-26
>> ID 275 top level 5 path
>> u1/linux/lvm+btrfs/storage/data/snapshots/2011-03-07 ID 276 top
>> level 5 path u1/linux/lvm+btrfs/storage/data/snapshots/2011-04-01
>> ID 277 top level 5 path u2/linux/lvm/linux/home/data ID 278 top
>> level 5 path u2/linux/lvm/linux/home/snapshots/2011-04-27 ID 279
>> top level 5 path u2/linux/lvm/linux/root/snapshots/2011-04-27 ID
>> 280 top level 5 path u2/linux/lvm/linux/var/snapshots/2011-04-27
>> ID 281 top level 5 path u3:10022/vm+xfs@u9/xvda1/g1/v4/data ID
>> 282 top level 5 path
>> u3:10022/vm+xfs@u9/xvda1/g1/v4/snapshots/2011-05-19 ID 283 top
>> level 5 path u5/vm+xfs@u9/xvda1/g1/v5/data ID 284 top level 5
>> path u6:10022/vm+xfs@u8/xvda1/g8/v3/data ID 286 top level 5 path
>> u5/vm+xfs@u9/xvda1/g1/v5/snapshots/2011-05-24 ID 287 top level
>> 285 path data ID 288 top level 5 path
>> u4/vm+xfs@u9/xvda1/g1/v1/data ID 289 top level 5 path
>> u4/vm+xfs@u9/xvda1/g1/v1/snapshots/2011-03-11 ID 290 top level 5
>> path u4/vm+xfs@u9/xvda1/g1/v2/data ID 291 top level 5 path
>> u4/vm+xfs@u9/xvda1/g1/v2/snapshots/2011-05-11 ID 292 top level 5
>> path u4/vm+xfs@u9/xvda1/g1/v1/snapshots/2011-05-11
> [...]
>> There is no "/backup/data" directory. There is however a
>> /backup/u6:10022/vm+xfs@u8/xvda1/g8/v3/snapshots/2011-03-30 that
>> contains the same thing as what I get if I mount the fs with
>> subvolid=287. And I did do a btrfs sub snap data
>> snapshots/2011-03/30 there.
>>
>> What could be the cause of that? How to fix it?
>>
>> In case that matters, there used to be more components in the
>> path of u6:10022/vm+xfs@u8/xvda1/g8/v3/data.
> [...]
>
> I tried deleting the
> /backup/u6:10022/vm+xfs@u8/xvda1/g8/v3/snapshots/2011-03-30
> subvolume (what seems to be id 287) and I get:
>
> # btrfs sub delete snapshots/2011-03-30 Delete subvolume
> '/backup/u6:10022/vm+xfs@u8/xvda1/g8/v3/snapshots/2011-03-30'
> ERROR: cannot delete
> '/backup/u6:10022/vm+xfs@u8/xvda1/g8/v3/snapshots/2011-03-30'
>
> With a strace, it tells me:
>
> ioctl(3, 0x5000940f, 0x7fffc7841a80) = -1 ENOTEMPTY (Directory
> not empty)
>
> Then I realised that there was a "data" directory in there and
> that snapshots/2011-03-30 was actually id 285 (which doesn't appear
> in the btrfs sub list) and "snapshots/2011-03-30/data" is id 287.
>
> What do those top-level IDs mean by the way?
The top-level ID associated with a subvolume is NOT the ID of this
particular subvolume but of the subvolume containing it. Since the
"root/initial" (sub-)volume has always ID 0, the subvolumes of "depth"
1 will all have top-level ID set to 0. You need those top-level IDs to
correctly mount a specific subvolume by name.

# mount /dev/dummy -o subvol=<subvolume>,subvolrootid=<top-level ID>
/mountpoint

Of course, you do need them, if you specify the subvolume to mount by
its ID.

Cheers,
Andreas Philipp

>
> Then I was able to delete snapshots/2011-03-30/data, but
> snapshots/2011-03-30 still didn't appear in the list.
>
> Then I was able to delete snapshots/2011-03-30 and recreate it,
> and this time it was fine.
>
> Still don't know what happened there.
>