qgroup: Add new subcommand to set/get drop_subtree_threshold

[kdave]

For convenience we should provide a command to set the threshold, right now it's something like echo 1 > /sys/fs/btrfs/$uuid/qgroups/drop_subtree_threshold where the uuid must be found by other means.

Related note: we also have per-filesystem quota_override, an command may be also good, with unified naming. We have the property command so current suggestion is to set it like that instead of adding set/get command to quota or qgroup.

[realwakka]

Hi, I sent a patch. I would appreciate it if you could review it. https://lore.kernel.org/linux-btrfs/[email protected]/

[realwakka]

Is this issue outdated?

[adam900710]

Kinda of. The upstream kernel now sets a default value 3, which is better than using any user space tool to set an initial value.