I think this has been fixed, but at one point, I used a log device with a one-time encryption key, thinking I didn't care that much about the data if there's a power loss. For the most part, ZFS didn't care either, but what it did care about was not mounting a pool with a missing disk. I had to edit metadata on the pool so it would mount with a different log device. Tools for this didn't exist--I was reading specs and source code, learning how ZFS know which devices belong in a pool.