On Tue, Aug 31, 2021 at 05:48:58PM +0800, Qu Wenruo wrote:

Btrfs qgroup has a long history of bringing huge performance penalty,
from subvolume dropping to balance.

Although we solved the problem for balance, but the subvolume dropping
problem is still unresolved, as we really need to do all the costly
backref for all the involved subtrees, or qgroup numbers will be
inconsistent.

But the performance penalty is sometimes too big, so big that it's
better just to disable qgroup, do the drop, then do the rescan.

This patchset will address the problem by introducing a user
configurable sysfs interface, to allow certain high subtree dropping to
mark qgroup inconsistent, and skip the whole accounting.

The following things are needed for this objective:

- New qgroups attributes

  Instead of plain qgroup kobjects, we need extra attributes like
  drop_subtree_threshold.

  This patchset will introduce two new attributes to the existing
  qgroups kobject:
  * qgroups_flags
    To indicate the qgroup status flags like ON, RESCAN, INCONSISTENT.

  * drop_subtree_threshold
    To show the subtree dropping level threshold.
    The default value is BTRFS_MAX_LEVEL (8), which means all subtree
    dropping will go through the qgroup accounting, while costly it will
    try to keep qgroup numbers as consistent as possible.

    Users can specify values like 3, meaning any subtree which is at
    level 3 or higher will mark qgroup inconsistent and skip all the
    costly accounting.

    This only affects subvolume dropping.

- Skip qgroup accounting when the numbers are already inconsistent

  But still keeps the qgroup relationship correct, thus users can keep
  its qgroup organization while do the rescan later.


This sysfs interface needs user space tools to monitor and set the
values for each btrfs.

Currently the target user space tool is snapper, which by default
utilizes qgroups for its space-aware snapshots reclaim mechanism.

This is an interesting approach, though I'm there are some usability
questions. First as a user, how do I know I need to use it?  The height
of the subvolume fs tree is not easily accessible.

The sysfs file is not protected in any way so multiple tools can decide
to set it to different values. And whether rescan is required or not
depends on the value so setting it.

It might be better to set the level (or a bit) to the subvol deletion
request, eg. a "fast" mode that would internally use maximum height 3 to
do slow deletion and anything else for fast leaving qgroup numbers
inconsistent.