On Tue, Sep 10, 2024 at 03:21:04PM +0930, Qu Wenruo wrote:
> Since commit 011b46c30476 ("btrfs: skip subtree scan if it's too high to
> avoid low stall in btrfs_commit_transaction()"), btrfs qgroup can
> automatically skip large subtree scan at the cost of marking qgroup
> inconsistent.
>
> It's designed to address the final performance problem of snapshot drop
> with qgroup enabled, but to be safe the default value is
> BTRFS_MAX_LEVEL, requiring a user space daemon to set a different value
> to make it work.
>
> I'd say it's not a good idea to rely on user space tool to set this
> default value, especially when some operations (snapshot dropping) can
> be triggered immediately after mount, leaving a very small window to
> that that sysfs interface.
>
> So instead of disabling this new feature by default, enable it with a
> low threshold (3), so that large subvolume tree drop at mount time won't
> cause huge qgroup workload.
Sounds like a sane idea to set it to some low value as default.
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
