Re: strange btrfs sub list output

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

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On 27.05.2011 13:30, Stephane Chazelas wrote:
> 2011-05-27 10:45:23 +0100, Hugo Mills: [...]
>>> How could a "subvolume 285" become a "top level"?
>>
>>> How does one get a subvolume with a top-level other than "5"?
>>
>> This just means that subvolume 287 was created (somewhere)
>> inside subvolume 285.
>>
>> Due to the way that the FS trees and subvolumes work, there's no
>> global namespace structure in btrfs; that is, there's no single
>> data structure that represents the entirety of the file/directory
>> hierarchy in the filesystem. Instead, it's broken up into these
>> sub-namespaces called subvolumes, and we only record parent/child
>> relationships for each subvolume separately. The "full path" you
>> get from "btrfs subv list" is reconstructed from that information
>> in userspace(*).
> [...]
>
> 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?

Thanks,
Andreas
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