[PATCH v6 0/4] btrfs: sysfs: set / query btrfs chunk size

[Stefan Roesch <shr@fb.com>]

The btrfs allocator is currently not ideal for all workloads. It tends
to suffer from overallocating data block groups and underallocating
metadata block groups. This results in filesystems becoming read-only
even though there is plenty of "free" space.

This is naturally confusing and distressing to users.

Patches:
1) Store the chunk size in the btrfs_space_info structure
2) Add a sysfs entry to read and write the above information
3) Add a sysfs entry to force a space allocation
4) Increase the default size of the metadata chunk allocation to 5GB
   for volumes greater than 50GB.

Testing:
  A new test is being added to the xfstest suite. For reference the
  corresponding patch has the title:
    [PATCH] btrfs: Test chunk allocation with different sizes

  In addition also manual testing has been performed.
    - Run xfstests with the changes and the new test. It does not
      show new diffs.
    - Test with storage devices 10G, 20G, 30G, 50G, 60G
      - Default allocation
      - Increase of chunk size
      - If the stripe size is > the free space, it allocates
        free space - 1MB. The 1MB is left as free space.
      - If the device has a storage size > 50G, it uses a 5GB
        chunk size for new allocations.

---
v6: - Update btrfs_force_chunk_alloc_store to use tree_root
      instead of extent_root.
V5: - Changed the field name in the btrfs_space_info struct from
      default_chunk_size to chunk_size and made it atomic
    - Removed the compute_chunk_size_zoned function
    - Added further checks when writing /sys/fs/btrfs/<id>/allocation/<type>/chunk_size
    - Removed the ability to query /sys/fs/btrfs/<id>/allocation/<type>/force_alloc_chunk

V4: - Patch email contained duplicate entries.

V3: - Rename sysfs entry from stripe_size to chunk_size
    - Remove max_chunk_size field in data structure btrfs_space_info
    - Rename max_stripe_size field to default_chunk_size in data
      structure btrfs_space_info
    - Remove max_chunk_size logic
    - Use stripe_size = chunk_size

V2:
   - Split the patch in 4 patches
   - Added checks for zone volumes in sysfs.c
   - Replaced the BUG() with ASSERT()
   - Changed if with fallthrough
   - Removed comments in space-info.h
--
Stefan Roesch (4):
  btrfs: store chunk size in space-info struct.
  btrfs: expose chunk size in sysfs.
  btrfs: add force_chunk_alloc sysfs entry to force allocation
  btrfs: increase metadata alloc size to 5GB for volumes > 50GB

 fs/btrfs/space-info.c |  41 ++++++++++++
 fs/btrfs/space-info.h |   3 +
 fs/btrfs/sysfs.c      | 152 ++++++++++++++++++++++++++++++++++++++++++
 fs/btrfs/volumes.c    |  28 +++-----
 4 files changed, 205 insertions(+), 19 deletions(-)


Signed-off-by: Stefan Roesch <shr@fb.com>
base-commit: 87c673b657a7e4784fb7274a77a8529712232d0c
-- 
2.30.2