command.
WARNING: Qgroup is not stable yet and will impact performance in current mainline
-kernel (v3.14 so far).
+kernel (v4.14).
QGROUP
------
Create a subvolume quota group.
+
For the '0/<subvolume id>' qgroup, a qgroup can be created even before the
-subvolume created.
+subvolume is created.
*destroy* <qgroupid> <path>::
Destroy a qgroup.
+
-If a qgroup is no isolated,which means it is a parent or child qgroup, it
-can't be destroyed.
+If a qgroup is not isolated, meaning it is a parent or child qgroup, then it
+can only be destroyed after the relationship is removed.
*limit* [options] <size>|none [<qgroupid>] <path>::
Limit the size of a qgroup to <size> or no limit in the btrfs filesystem