Btrfs contains different implementations of a free space cache. The default currently is
space_cache=v1. A newer tree-based cache can be activated by the mount option
space_cache=v2. As soon as the new version has been used once, it will automatically be used for any future mount. But how do you figure out which version your file system is actually using?
There are multiple ways to determine the used space cache version. The easiest one is to just mount the file system and look into the kernel log. If the default space cache v1 is used, it will say something like
BTRFS info (device yourdevice): disk space caching is enabled
However, if it already has been configured to use the new space cache v2, it will say something like
BTRFS info (device yourdevice): using free space tree
One can also determine the used space cache version without mounting the file system by peeking into the superblock:
btrfs inspect-internal dump-super /dev/yourdev
If the file system already uses
compat_ro_flags will contain the flag
For completeness: When playing with
space_cache=v2, be aware of the risks (see the remarks on the space_cache mount option in btrfs(5)).