Comment by dapperdrake

zpool-replace(8)

https://openzfs.github.io/openzfs-docs/man/master/8/zpool-re...

ZFS got this right. MDADM on ext4 somehow did not.

EDIT: ext4 on MDADM.

How ZFS gets this right: zfs(8) on zpool(8). So, basically, ext4 and mdadm can talk to each other.