mount snapshot by object-id

mount snapshot by object-id

[Michael Niederle]

Hi!

I'm currently writing a btrfs-rescue tool and therefor began to study the
btrfs-on-disk structures in detail.

The root tree contains a ROOT_ITEM entry for *every* subvolume in the whole
file system, but only DIR_ITEM entries for subvolumes that were created in the
root directory of the filesystem.

If we have a destroyed root directory, there is no way to access subvolumes
stored deeper in the fs-tree by name. It might be nice to have the ability to
mount a subvolume by object-id. A simple tool could list all available
subvolumes with their object-ids, generation numbers, and so on.

Greetings, Michael

Re: mount snapshot by object-id

[Calvin Walton]

On Thu, 2010-12-09 at 14:51 +0100, Michael Niederle wrote:
> Hi!
> 
> I'm currently writing a btrfs-rescue tool and therefor began to study the
> btrfs-on-disk structures in detail.
> 
> The root tree contains a ROOT_ITEM entry for *every* subvolume in the whole
> file system, but only DIR_ITEM entries for subvolumes that were created in the
> root directory of the filesystem.
> 
> If we have a destroyed root directory, there is no way to access subvolumes
> stored deeper in the fs-tree by name. It might be nice to have the ability to
> mount a subvolume by object-id. A simple tool could list all available
> subvolumes with their object-ids, generation numbers, and so on.

Unless I misunderstand something, this is actually already possible;
just poorly documented. You can use the 'subvolid=' mount option to
mount by object id; the object id can be found using the tool
btrfs subvolume list <filesystem>

The subvolid= mount option was missing from the wiki, so I've just added
it:
https://btrfs.wiki.kernel.org/index.php/Getting_started#Mount_Options

-- 
Calvin Walton <calvin.walton@gmail.com>

Re: mount snapshot by object-id

[Dipl.-Ing. Michael Niederle]

Hi, Calvin!

Thanks a lot for this information and for updating the wiki!

The option works - on healthy disks ...

I will continue writing my rescue-tool. I also wrote a btrfs_subvolumes command
that displays all subvolumes of an unmounted filesystem. This helps a lot if
mounting the filesystem leads to a crash!

Greetings, Michael