Minimal adaptation of zfs-auto-snapshot to btrfs. It's in a "works for me using the provided cronjobs" state; test in a virtual machine before even dreaming of using it, and consider using https://github.com/nachoparker/btrfs-snp instead! In particular, something I have NOT tested is https://github.com/csirac2/snazzer/commit/50bf4f22a7bf267180556a3fe4af5b16ea0f2dda ie what happens if the subvolume is owned by someone other than root. Also at present there is no way to disable snapshots for /var/cache or whatever.