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

[David Sterba <dsterba@suse.cz>]

On Wed, May 01, 2024 at 07:57:08AM +0930, Qu Wenruo wrote:
> 
> 
> 在 2024/5/1 07:48, Boris Burkov 写道:
> > On Wed, May 01, 2024 at 07:35:09AM +0930, Qu Wenruo wrote:
> [...]
> >>>
> >>> 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.
> >
> > This was my initial thought too, but your compat bit idea is interesting
> > since it persists? I vote sysfs since it has good
> > infrastructure/momentum already for similar config.
> 
> Sysfs is another way which we are already utilizing for qgroups, like
> drop_subtree_threshold.
> 
> 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.