Install FreeBSD 11 on ZFS mirror (GPT or MBR)

Finish installation

On the last installation screen choose shell again when asked.

Create system init configs

# echo 'zfs_load="YES"' >> /boot/loader.conf
# echo 'vfs.root.mountfrom="zfs:system"' >> /boot/loader.conf
# echo 'zfs_enable="YES"' >> /etc/rc.conf

Create fstab file

/etc/fstab (GPT)

# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/cd0                /media/cdrom    cd9660  ro,noauto       0       0
/dev/da0s1              /media/flash    msdosfs rw,sync,noauto,longnames,-Lru_RU.UTF-8 0 0
/dev/ufs/share          /share          ufs     rw              1       2

/etc/fstab (MBR)

# Device                Mountpoint      FStype  Options         Dump    Pass#
/dev/cd0                /media/cdrom    cd9660  ro,noauto       0       0
/dev/da0s1              /media/flash    msdosfs rw,sync,noauto,longnames,-Lru_RU.UTF-8 0 0
/dev/ufs/share          /share          ufs     rw              1       2

ZFS filesystems does not need fstab entries. Adjust flash mount options as you want.

Create mount points:

# cd /media
# mkdir cdrom flash
# cd /
# mkdir share

I have only one UFS filesystem — /share. If you don't have any then skip the step. Or create mountpoints and fstab entries for filesystems you have.

Note that I use /dev/ufs/share as the device. If your UFS2 filesystem does not have a label than it's time to create one:

# tunefs -L share /dev/ada3s1d

Reboot into your new system

Type exit, finish the installation and reboot. Set BIOS to boot from one of the mirrored drives.

Login to the system and:

# zfs set readonly=on system/var/empty
# rm /etc/motd

Create swap (eight gigabytes here — change it):

# zfs create -V 8G -o org.freebsd:swap=on -o checksum=off -o compression=off \
  -o dedup=off -o sync=disabled -o primarycache=none system/swap
# swapon /dev/zvol/system/swap

Now you can start configuring your new system. Have a look at Initial configuration article.