Unexpected message 'clearing space cache v1' with v2 enabled

[kdave]

https://lore.kernel.org/linux-btrfs/38e1b33.df69821a.17cd0454b6b@tnonline.net

Creating a filesystem with enabled free space tree by default, there's a message in kernel log "cleaning free space cache v1". That's a bit unexpected but otherwise seems harmless.

[kdave]

$ ./mkfs.btrfs --csum xxhash img
btrfs-progs v5.14.91 
See http://btrfs.wiki.kernel.org for more information.

NOTE: several default settings have changed in version 5.15, please make sure
      this does not affect your deployments:
      - DUP for metadata (-m dup)
      - enabled no-holes (-O no-holes)
      - enabled free-space-tree (-R free-space-tree)

Label:              (null)
UUID:               0fece3ad-b8cf-4038-bb74-9bfa0e4dfd29
Node size:          16384
Sector size:        4096
Filesystem size:    2.00GiB
Block group profiles:
  Data:             single            8.00MiB
  Metadata:         dup             102.38MiB
  System:           dup               8.00MiB
SSD detected:       no
Zoned device:       no
Incompat features:  extref, skinny-metadata, no-holes
Runtime features:   free-space-tree
Checksum:           xxhash64
Number of devices:  1
Devices:
   ID        SIZE  PATH
    1     2.00GiB  img

Kernel log:

[6052398.182353] BTRFS: device fsid 0fece3ad-b8cf-4038-bb74-9bfa0e4dfd29 devid 1 transid 6 /dev/loop0 scanned by mount (29702)
[6052398.183319] BTRFS info (device loop0): using free space tree
[6052398.183324] BTRFS info (device loop0): has skinny extents
[6052398.183327] BTRFS info (device loop0): flagging fs with big metadata feature
[6052398.189916] BTRFS info (device loop0): enabling ssd optimizations
[6052398.190060] BTRFS info (device loop0): cleaning free space cache v1
[6052398.198533] BTRFS info (device loop0): checking UUID tree

[wangyugui]

the reason:

  # mkfs.btrfs -R free-space-tree test.img                                                                                                                                                                           
  # btrfs inspect-internal dump-super test.img | grep cache_generation                                                                                                                                               

0 is expected, but -1 is returned

[kdave]

Duplicate of #414, please continue there.