Re: strange btrfs sub list output

[C Anthony Risinger <anthony@xtfx.me>]

On Tue, May 31, 2011 at 5:00 AM, Stephane Chazelas
<stephane_chazelas@yahoo.fr> wrote:
> 2011-05-27 13:49:52 +0200, Andreas Philipp:
> [...]
>> > Thanks, I can understand that. What I don't get is how one creates
>> > a subvol with a top-level other than 5. I might be missing the
>> > obvious, though.
>> >
>> > If I do:
>> >
>> > btrfs sub create A btrfs sub create A/B btrfs sub snap A A/B/C
>> >
>> > A, A/B, A/B/C have their top-level being 5. How would I get a new
>> > snapshot to be a child of A/B for instance?
>> >
>> > In my case, 285, was not appearing in the btrfs sub list output,
>> > 287 was a child of 285 with path "data" while all I did was create
>> > a snapshot of 284 (path u6:10022/vm+xfs@u8/xvda1/g8/v3/data in vol
>> > 5) in u6:10022/vm+xfs@u8/xvda1/g8/v3/snapshots/2011-03-30
>> >
>> > So I did manage to get a volume with a parent other than 5, but I
>> > did not ask for it.
> [...]
>> Reconsidering the explanations on btrfs subvolume list in this thread
>> I get the impression that a line in the output of btrfs subvolume list
>> with top level other than 5 indicates that the backrefs from one
>> subvolume to its parent are broken.
>>
>> What's your opinion on this?
> [...]
>
> Given that I don't really get what the parent-child relationship
> means in that context, I can't really comment.
>
> In effect, the snapshot had been created and was attached to the
> right directory (but didn't appear in the sub list), and there
> was an additional "data" volume that I had not asked for nor
> created that had the snapshot above as parent and that did
> appear in the sub list.
>
> It pretty much looks like a bug to me, I'd like to understand
> more so that I can maybe try and avoid running into it again.

i'm actually really interested in the conclusion to this thread
because i _want_ to create subvols with a new parent ... i didn't
realize this wasn't possible (nor the mount option) until reading this
thread.  this would give me a little more flexibility with initcpio
hooks and the like vs. packing the btrfs root with tons of hidden
files [subvols] or using IDs directly ...

i tried absolutely everything i could think of to reproduce this but
all subvols ended up having a top level id of `5`.

... so, is there any known way to _purposefully_ create parented
subvols with the current tools?

-- 

C Anthony