[sstephant]

While looking up for ZFS ncopies option to see what it actually does I stumbled onto this blog post:

https://jrs-s.net/2016/05/02/zfs-copies-equals-n/

"zfs: copies=n is not a substitute for device redundancy!"

[tw04]

Correct, because copies may place all copies onto the same disk.

The point of copies is belt + suspenders: you both have your typical raid protection as well as two individual copies of the file so if one copy becomes corrupted the second can be referenced.

Additionally it's useful in the case you have a single disk (like a laptop). That way if you end up with a corrupted block you can still recover your file.

[brnt]

No, but that's not the goal of parity files either. It protects against bitrot.