Yep, that works:

    Logical  Sector size:                   512 bytes
    Physical Sector size:                  4096 bytes

I'm guessing that's not very nice. My ZFS pool was created with ashift 9 (this is 2^9=512 bytes), when it should be 12 (2^12=4096). I will have to copy everything off and back on again.

For everyone who wants to check, and because I couldn't find info on it, run:

    zdb | grep ashift

And see if it's 9 or 12.