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

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

在 2024/4/30 20:29, David Sterba 写道:
> On Tue, Apr 30, 2024 at 07:35:11AM +0930, Qu Wenruo wrote:
>>
>>
>> 在 2024/4/30 02:01, David Sterba 写道:
>>> On Mon, Apr 29, 2024 at 06:13:33AM -0700, Boris Burkov wrote:
>>>> I support the auto deletion in the kernel as you propose, I think it
>>>> just makes sense. Who wants stale, empty qgroups around that aren't
>>>> attached to any subvol? I suppose that with the drop_thresh thing, it is
>>>> possible some parent qgroup still reflects the usage until the next full
>>>> scan?
>>>
>>> The stale qgroups have been out for a long time so removing them after
>>> subvolume deletion is changing default behaviour, this always breaks
>>> somebody's scripts or tools.
>>
>> If needed I can introduce a compat bit (the first one), to tell the
>> behavior difference.
>>
>> And if we go the compat bit way, I believe it can be the first example
>> on how to do a kernel behavior change correctly without breaking any
>> user space assumption.
>
> I don't see how a compat bit would work here, we use them for feature
> compatibility and for general access to data (full or read-only). What
> we do with individual behavioral changes are sysfs files. They're
> detectable by scripts and can be also used for configuration. In this
> case enabling/disabling autoclean of the qgroups.
>

I mean the compat bit, which is fully empty now.

The new bit would be something like BTRFS_QGROUP_AUTO_REMOVAL, with that
set, btrfs would auto remove any stale qgroups (for regular qgroups though).

Without that, it would be as usual (no auto removal).

Since this doesn't cause any on-disk change, it does not needs compat-ro
nor incompat.

Thanks,
Qu