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

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

在 2024/5/3 22:16, David Sterba 写道:
> On Fri, May 03, 2024 at 06:59:03AM +0930, Qu Wenruo wrote:
>>>> The problem is as mentioned already, it's not persistent, thus it needs
>>>> a user space daemon to set it for every fs after mount.
>>>
>>> My idea of using sysfs is to export the information that the
>>> autocleaning feature is present and if we make it on by default then
>>> there's no need for additional step to enable it. The feedback about
>>> that was that it should have been default so we're going to make that
>>> change, but with sysfs export also provide a fallback to disable it in
>>> case it breaks things for somebody.
>>>
>>>> I'm totally fine to go sysfs for now, but I really hope to a persistent
>>>> solution.
>>>> Maybe a dedicated config tree?
>>>
>>> No, we already have a way to store data in the trees or in the
>>> properties so no new tree.
>>
>> That means a on-disk format change.
>> IIRC everytime we introduce a new TEMP objectid, it should at least be
>> compat or compat_ro bit change.
>>
>> Or older kernel won't understand nor follow the new TEMP key.
>
> This is not necessarily a problem, if older kernel does not understand
> the key it'll skip it. The search treats the keys as numbers. If there's
> something more associated with the additional data that would prevent
> mount then a compat bit would make sense.
>
> The compat_ro does not make sense here, the same filesystem can be
> mounted and data read or written, the only difference is that qgroups
> are also removed. How is that incompatible on the same level as other
> features like BGT?

That's why BGT is compat_ro, not compat.
And exactly why I want to push a new compat bit just for qgroup auto reap.

Thanks,
Qu