Re: [PATCH v2 2/2] btrfs: automatically remove the subvolume qgroup

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

在 2024/4/24 22:11, David Sterba 写道:
> On Fri, Apr 19, 2024 at 07:16:53PM +0930, Qu Wenruo wrote:
>> Currently if we fully removed a subvolume (not only unlinked, but fully
>> dropped its root item), its qgroup would not be removed.
>>
>> Thus we have "btrfs qgroup clear-stale" to handle such 0 level qgroups.
>
> There's also an option 'btrfs subvolume delete --delete-qgroup' that
> does that and is going to be default in 6.9. With this kernel change it
> would break the behaviour of the --no-delete-qgroup, which is there for
> the case something depends on that.  For now I'd rather postpone
> changing the kernel behaviour.
>

A quick glance of the --delete-qgroup shows it won't work as expected at
all.

Firstly, the qgroup delete requires the qgroup numbers to be 0.
Meanwhile qgroup numbers can only be 0 after 1) the full subvolume has
been dropped 2) a transaction is committed to reflect the qgroup numbers.

Both situation is only handled in my patchset, thus this means for a lot
of cases it won't work at all.

Furthermore, there is the drop_subtree_threshold thing, which can mark
qgroup inconsistent and skip accounting, making the target subvolume's
qgroup numbers never fall back to 0 (until next rescan).

So I'm afraid the --delete-qgroup won't work until the 1/2 patch get
merged (allowing deleting qgroups as long as the target subvolume is gone).

Thanks,
Qu