btrfs: disable v1 space cache

V2 space cache has been the default mkfs option since btrfs-progs v5.15,
and commit 1e7bec1f7d65 ("btrfs: emit a warning about space cache v1
being deprecated") has already added a warning to show v1 space cache
has been deprecated.

It has been long enough that we should remove v1 space cache completely.

As the first step, disable v1 space cache by:

- Make "space_cache" mount option fallback to "nospace_cache"

- Make "space_cache=v1" fall back to "nospace_cache"

  This is safer than forcing "space_cache=v2", as forcing v2 cache
  requires removal of v1 cache and regenerating v2 cache.
  Such operation can be slow, and takes extra metadata space, thus
  it is not always safe for existing filesystems.

With this done, v1 cache mount will always fallback to nospace cache,
and mount option will not be able to force v1 space cache usage.

For example, even for a fs with v1 cache:

  # btrfs ins dump-super test.img
  superblock: bytenr=65536, device=test.img
  ---------------------------------------------------------
  csum_type		0 (crc32c)
  csum_size		4
  csum			0xdce44b2c [match]
  bytenr		65536
  flags			0x1
  			( WRITTEN )
  magic			_BHRfS_M [match]
  fsid			7d7c3bba-8211-4206-868d-10eedd5703f8
  metadata_uuid		00000000-0000-0000-0000-000000000000
  label
  generation		9
  root			30605312
  [...]
  compat_ro_flags	0x0                     <<< No FST feature
  incompat_flags	0x361
  			( MIXED_BACKREF |
  			  BIG_METADATA |
  			  EXTENDED_IREF |
  			  SKINNY_METADATA |
  			  NO_HOLES )
  cache_generation	9                       <<< Matches generation
  uuid_tree_generation	9

Attempting to mount it will lead to no space cache other than v1 space cache:

  # mount test.img /mnt/btrfs
  # dmesg -t | tail -n 5
  BTRFS: device fsid 7d7c3bba-8211-4206-868d-10eedd5703f8 devid 1 transid 9 /dev/loop0 (7:0) scanned by mount (1264)
  BTRFS info (device loop0): first mount of filesystem 7d7c3bba-8211-4206-868d-10eedd5703f8
  BTRFS info (device loop0): using crc32c checksum algorithm
  BTRFS info (device loop0): turning on async discard
  BTRFS info (device loop0): last unmount of filesystem 7d7c3bba-8211-4206-868d-10eedd5703f8

Even forcing v1 cache will not work, but fallback to the usual
nospace_cache:

  # mount test.img -o space_cache=v1 /mnt/btrfs
  # dmesg -t | tail -n 6
  BTRFS warning: v1 space cache is deprecated, fallback to no space cache
  BTRFS: device fsid 7d7c3bba-8211-4206-868d-10eedd5703f8 devid 1 transid 9 /dev/loop0 (7:0) scanned by mount (1264)
  BTRFS info (device loop0): first mount of filesystem 7d7c3bba-8211-4206-868d-10eedd5703f8
  BTRFS info (device loop0): using crc32c checksum algorithm
  BTRFS info (device loop0): turning on async discard
  BTRFS info (device loop0): last unmount of filesystem 7d7c3bba-8211-4206-868d-10eedd5703f8

And there will be no way to force converting a v2 cache back to v1, such
attempt will only clear free space tree and fallback to no space cache.

  # mkfs.btrfs -f -O fst,^bgt test.img
  # mount -o clear_cache,space_cache=v1 test.img /mnt/btrfs
  # dmesg -t | tail -n 11
  BTRFS warning: v1 space cache is deprecated, fallback to no space cache
  BTRFS: device fsid f59daad2-3ab5-4f33-b752-a36cfb09b674 devid 1 transid 8 /dev/loop0 (7:0) scanned by mount (1419)
  BTRFS info (device loop0): first mount of filesystem f59daad2-3ab5-4f33-b752-a36cfb09b674
  BTRFS info (device loop0): using crc32c checksum algorithm
  BTRFS info (device loop0): rebuilding free space tree
  BTRFS info (device loop0): disabling free space tree
  BTRFS info (device loop0): clearing compat-ro feature flag for FREE_SPACE_TREE (0x1)
  BTRFS info (device loop0): clearing compat-ro feature flag for FREE_SPACE_TREE_VALID (0x2)
  BTRFS info (device loop0): checking UUID tree
  BTRFS info (device loop0): turning on async discard
  BTRFS info (device loop0): force clearing of disk cache
  # mount | grep /mnt/btrfs
  /mnt/test.img on /mnt/btrfs type btrfs (rw,relatime,discard=async,nospace_cache,subvolid=5,subvol=/)

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

0 files changed, 0 insertions, 0 deletions