Re: strange btrfs sub list output

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

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On 27.05.2011 11:45, Hugo Mills wrote:
> On Fri, May 27, 2011 at 10:30:29AM +0100, Stephane Chazelas wrote:
>> 2011-05-27 10:12:24 +0100, Hugo Mills:
>> [skipped useful clarification]
>>>
>>> That's all rather dense, and probably too much information. Hope
>>> it's helpful, though.
>> [...]
>>
>> It is, thanks.
>>
>> How would one end up in a situation where the output of "btrfs
>> sub list ." has:
>>
>> ID 287 top level 285 path data
>>
>> 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(*).
>
> Hugo.
>
> (*) Here's how it does it:
>
> The userspace tool gets a list of every subvolume by looking at the FS
> tree. It uses the corresponding back-refs to get the inode that
> represents each of those FS trees inside its parent:
>
> Subvol inode in subvol
> 256 991 5
> 257 896 256
> 258 1073 257
>
> From the inode numbers, it can then recursively walk back up the
> directory path to the top of each subvolume:
>
> Subvol inode in subvol relative path
> 256 991 5 henry
> 257 896 256 edward/mary
> 258 1073 257 elizabeth
>
> From that, it can then reconstruct the full pathnames, by walking back
> up the subvolume tree:
>
> subvol 258 is elizabeth in 257
> is edward/mary/elizabeth in 256
> is henry/edward/mary/elizabeth in 5
Just one (hopefully) short question: A line in the ouput of btrfs
subvolume list like
ID 257 top level 5 path test1/test1.1
says that the subvolume with name test1.1 (the last segment of the
path) and ID 257 has the path test1/test1.1 starting at the top level
subvolume which has ID 5 ?

Thanks,
Andreas
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