btrfs-progs: docs: update the stability on btrfs-quota

The core design of btrfs qgroup mode is to track the ownership of every
extent accurately, but that is conflicting with snapshots, which can
change the ownership of tons of extents in a very short time, during
snapshot creation and deletion.

Although qgroup mode has several workarounds, it is not fully resolved
and still introduces extra costs, e.g. requires frequent rescan,
breaking limits etc.

So make it more clear that qgroup mode is not recommended until the end
user has a rigid subvolume layout, e.g. no more new subvolume/snapshots,
nor deletion of any existing subvolume.

Signed-off-by: Qu Wenruo <wqu@suse.com>

Author: adam900710

Documentation/btrfs-quota.rst

@@ -22,12 +22,23 @@ of a btrfs filesystem. The quota groups (qgroups) are managed by the subcommand
 STABILITY AND PERFORMANCE IMPLICATIONS
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-The qgroup implementation is considered reasonably stable for daily use and has
-been enabled in various distributions.
+The qgroup mode is considered not recommended for daily usage, unless there is
+no planned new snapshot/subvolume creation and deletion.
 
-When quotas are activated, they affect all extent processing, which takes a
-performance hit. Activation of qgroups is not recommended unless the user
-intends to actually use them.
+The core design of qgroup mode and snapshot are not compatible from day one, and
+are the cause of all kinds of performance problems.
+
+When qgroup mode is activated, it affects all extent processing, which takes a
+performance hit. Operations that modify a whole subvolume/snapshot in one go,
+which include snapshot creation and subvolume deletion, are heavily affected and
+can cause a system hang due to the heavy load.
+
+Although the kernel is taking several workarounds, the problem is not fully resolved
+and has extra costs, e.g. marking qgroup inconsistent, breaking limits and
+requiring extra rescan.
+
+Activation of qgroups is not recommended unless the user intends to actually use them,
+and the usage does not involve new subvolume/snapshot.
 
 .. _man-quota-hierarchical-quota-group-concepts: