[PATCH v2 0/2] btrfs: ENOSPC fixes for unused block groups

[Boris Burkov <boris@bur.io>]

A few fixes for system chunk allocation ENOSPCs caused by a sudden influx of a
large number of unused block groups in a filesystem with no free dev extents.

In practice, I saw this on a fully allocated fs with 30TB of space, a 4MiB
system chunk (single metadata mode) and ~10k unused data BGs.

- The first patch tries to avoid the condition by allocating earlier for the
  system chunk.

- The second patch eagerly commits a single deletion when the system chunk is
  at risk.

More details in the patches themselves.

Changelog:
v2:
- refactored some names and the flow in patch 1 to make it more clear.


Boris Burkov (2):
  btrfs: allocate additional SYSTEM space earlier
  btrfs: commit after deleting an unused block group if system space is
    low

 fs/btrfs/block-group.c | 59 +++++++++++++++++++++++++++++++++++-------
 1 file changed, 49 insertions(+), 10 deletions(-)

-- 
2.55.0