To find the used snapshot space, run:
zfs list -o space
To delete all ZFS snapshots, run:
zfs list -H -o name -t snapshot | xargs -n1 zfs destroy
Failed to delete dataset: cannot destroy snapshot dataset is busy
zfs holds raid/Dataset@auto-20190405.2100-6m"
raid/Dataset@auto-20190405.2100-6m freenas:repl Mon Jun 13 21:28 2016
zfs release -r freenas:repl raid/Dataset@auto-20190405.2100-6m