Ok, this is a long post but a useful one.  This is how to make freebsd boot from a ZFS volume (whether it be raid0, raid5 or raid6).  The freebsd installer doesn’t support anything exotic so we have to do this manually.

If you’re using FreeBSD 9.0, then follow the guide at https://www.dan.me.uk/blog/2012/01/22/booting-from-zfs-raid0156-in-freebsd-9-0-release/

First, grab yourself a copy of DVD1 iso or the memory stick image and boot from it.  No other boot image will work – it MUST be the DVD or memory stick image!

Once you’ve booted into the installer and chosen your country and keyboard layouts, go to the Fixit menu and choose either CDROM/DVD or USB depending on the installation media you used.  This will open up a terminal window into a live filesystem booted from the DVD/USB.

For my example, i’m going to build a RAID5 array on disks da0 da1 and da2.

First, we need to remove any existing GPT partition info from the disks – ignore the ‘invalid argument’ message if you get it at this stage:

gpart destroy da0
gpart destroy da1
gpart destroy da2

Now we need to initialise the GPT partitions on each disk:

gpart create -s gpt da0
gpart create -s gpt da1
gpart create -s gpt da2

We will now make a boot (64KB) and ZFS (remaining space) partition on each disk in turn:

gpart add -s 128 -t freebsd-boot da0
gpart add -s 128 -t freebsd-boot da1
gpart add -s 128 -t freebsd-boot da2

gpart add -t freebsd-zfs -l disk0 da0
gpart add -t freebsd-zfs -l disk1 da1
gpart add -t freebsd-zfs -l disk2 da2

And now we have to install the protected MBR boot code into all the drives:

gpart bootcode -b /mnt2/boot/pmbr -p /mnt2/boot/gptzfsboot -i 1 da0
gpart bootcode -b /mnt2/boot/pmbr -p /mnt2/boot/gptzfsboot -i 1 da1
gpart bootcode -b /mnt2/boot/pmbr -p /mnt2/boot/gptzfsboot -i 1 da2

Now that we’ve configured the disks, we need to load the ZFS kernel modules from the CD so that we can build ZFS volumes:

kldload /mnt2/boot/kernel/opensolaris.ko
kldload /mnt2/boot/kernel/zfs.ko

And create a ZFS pool.  If you want a RAID6 volume, choose raidz2 instead of raidz1 here.  If you want a mirror, use mirror or if you want RAID0 (or single disk) just omit the raidz1 completely:

zpool create zroot raidz1 /dev/gpt/disk0 /dev/gpt/disk1 /dev/gpt/disk2
zpool set bootfs=zroot zroot

Ok, now we’ve made our ZFS pool (and it’s currently mounted at /zroot/) – we need to make all our filesystems on it… this is complicated, but here we go:

zfs set checksum=fletcher4 zroot
zfs create -o compression=on -o exec=on -o setuid=off zroot/tmp
chmod 1777 /zroot/tmp
zfs create zroot/usr
zfs create zroot/usr/home
cd /zroot; ln -s /usr/home home
zfs create -o compression=lzjb -o setuid=off zroot/usr/ports
zfs create -o compression=off -o exec=off -o setuid=off zroot/usr/ports/distfiles
zfs create -o compression=off -o exec=off -o setuid=off zroot/usr/ports/packages
zfs create zroot/var
zfs create -o compression=lzjb -o exec=off -o setuid=off zroot/var/crash
zfs create -o exec=off -o setuid=off zroot/var/db
zfs create -o compression=lzjb -o exec=on -o setuid=off zroot/var/db/pkg
zfs create -o exec=off -o setuid=off zroot/var/empty
zfs create -o compression=lzjb -o exec=off -o setuid=off zroot/var/log
zfs create -o compression=gzip -o exec=off -o setuid=off zroot/var/mail
zfs create -o exec=off -o setuid=off zroot/var/run
zfs create -o compression=lzjb -o exec=on -o setuid=off zroot/var/tmp
chmod 1777 /zroot/var/tmp

Now we’re ready to install FreeBSD onto the new ZFS partitions.  We’re going to install the base, manual pages, all sources and a generic kernel – this takes some time so be patient…

cd /dist/8.1-RELEASE/
export DESTDIR=/zroot
for dir in base manpages ; do (cd $dir ; ./install.sh) ; done
cd src ; ./install.sh all
cd ../kernels ; ./install.sh generic
cd /zroot/boot ; cp -Rlp GENERIC/* /zroot/boot/kernel/

Now we need to set /var/empty to readonly:

zfs set readonly=on zroot/var/empty

And now we’re ready to configure the installation.  To make things easier, we will chroot into the environment:

chroot /zroot

We need to setup an initial /etc/rc.conf which will mount all ZFS filesystems:

echo ‘zfs_enable=”YES”‘ > /etc/rc.conf

And an initial /boot/loader.conf that will load the ZFS modules and set our root mountpoint:

echo ‘vfs.zfs.prefetch_disable=”1″‘ > /boot/loader.conf
echo ‘vfs.root.mountfrom=”zfs:zroot”‘ >> /boot/loader.conf
echo ‘zfs_load=”YES”‘ >> /boot/loader.conf

Now you can set your root password:

passwd root

And configure your timezone:

tzsetup

And setup a dummy aliases file for sendmail to keep it quiet 😉

cd /etc/mail
make aliases

You can do other configuration here, like adding a user etc – but when you’re done we can exit the environment:

exit

Now, we need to export our ZFS configuration (and reimport it) so we can save out the cache file:

mkdir /boot/zfs
cd /boot/zfs
zpool export zroot && zpool import zroot
cp /boot/zfs/zpool.cache /zroot/boot/zfs/zpool.cache

We now create an empty /etc/fstab file as follows:

touch /zroot/etc/fstab

This is the tricky part, we need to unmount the ZFS partitions and re-assign their mountpoints for the root filesystems:

export LD_LIBRARY_PATH=/mnt2/lib
zfs unmount -a
zfs set mountpoint=legacy zroot
zfs set mountpoint=/tmp zroot/tmp
zfs set mountpoint=/usr zroot/usr
zfs set mountpoint=/var zroot/var

Now we can exit the fixit shell, remove the media and reboot the computer.  Do this as soon as you can.

The computer should reboot into a ZFS-based filesystem, booted from a software RAID array on fully protected disks.

Once it’s booted, you can login and run sysinstall to configure other options like networking and startup programs (like SSH!)

Enjoy!