Why did rebooting cause one side of my ZFS mirror to become UNAVAIL?

This is a udev issue that seems to be specific to Debian and Ubuntu variants. Most of my ZFS on Linux work is with CentOS/RHEL.

Similar threads on the ZFS discussion list have mentioned this.

See:
scsi and ata entries for same hard drive under /dev/disk/by-id
and
ZFS on Linux/Ubuntu: Help importing a zpool after Ubuntu upgrade from 13.04 to 13.10, device IDs have changed

I'm not sure what the most deterministic pool device approach for Debian/Ubuntu systems is. For RHEL, I prefer to use device WWNs on general pool devices. But other times, the device name/serial is useful, too. But udev should be able to keep all of this in check.

# zpool status
  pool: vol1
 state: ONLINE
  scan: scrub repaired 0 in 0h32m with 0 errors on Sun Feb 16 17:34:42 2014
config:

        NAME                        STATE     READ WRITE CKSUM
        vol1                        ONLINE       0     0     0
          mirror-0                  ONLINE       0     0     0
            wwn-0x500000e014609480  ONLINE       0     0     0
            wwn-0x500000e0146097d0  ONLINE       0     0     0
          mirror-1                  ONLINE       0     0     0
            wwn-0x500000e0146090c0  ONLINE       0     0     0
            wwn-0x500000e01460fd60  ONLINE       0     0     0