In chapter “11.7.1 ZFS” the zfs mirrored pool is created with the command:
zpool create backup /dev/sdb /dev/sdc /dev/sdd cache /dev/sde -m /media/BACKUP
This is wrong because “mirror” is missing. The correct command is:
zpool create backup mirror /dev/sdb /dev/sdc /dev/sdd cache /dev/sde -m /media/BACKUP