Compression can only be set '''per volume'''. So if you set different "levels" of compression for different mounts in the fstab, only the first "level" will be applied. [https://btrfs.readthedocs.io/en/latest/Compression.html#how-to-enable-compression see@btrfs.readthedocs.io]
You can turn on compression at any time by changing the mount options in the fstab. After the next reboot, all newly written data from then on will be automatically compressed. Compression factors of 2:1 on average are not uncommon.
{{BoxWarning|Grub needs to load the kernel and initrd|When you use compression on kernel, initrd, or grub config files, '''grub''' needs to decompress these files. Otherwise you will '''not be able to boot'''. GRUB introduced '''zstd''' support in '''2.04.''' Maybe you need to update grub '''and''' reinstall it}}
You can turn on compression at any time by changing the mount options in the fstab. After the next reboot, all newly written data from then on will be automatically compressed. Compression factors of 2:1 on average are not uncommon.
@see mount-options