I think the primary advantage is that ZFS collapses all the standard filesystem abstractions. With mdadm or hardware raid you have a raid controller (which cou

I think the primary advantage is that ZFS collapses all the standard filesystem abstractions. With mdadm or hardware raid you have a raid controller (which could be mdadm), volume manager (i.e. lvm), and filesystem (ext4, xfs, etc). ZFS combines all of that into one. It's really a different philosophy, but means that things like creating a new filesystem is almost instant (and CoW, snapshots, replication are all easy - although perhaps that's possible with the traditional abstractions as well).