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

[David Sterba <dsterba@suse.cz>]

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?