Auto-pruning old snapshots when running out of storage

[nickcalyx]

Apple's Time machine ( which i've always thought was an elegant example to study ) works this way :

1. Initially it makes a full backup
2. after that, it makes incrementals each hour ( in our case it could be more time )
3. When the disk starts to run out of space, Time machine starts deleting the oldest snapshots

I'd like to propose that we find a way to auto-prune ( after a opt-out of pruning notification ? ) old snapshots when the storage is running out of space