Dual-boot Gentoo and FreeBSD zfs

This article suppose that you have Gentoo installed on zfs already.

Also consider this how-to as highly experimental and check all the commands that you know what are you doing.

Requirements (starting point)

Boot partition on ext2 (min 500MB).
grub:0 or grub:2 installed on boot

Update your zfs layout to something like this:

rpool                          47,1G  10,9G   160K  /rpool
rpool/GENTOO                   15,6G  10,9G   168K  /rpool/GENTOO
rpool/GENTOO/distfiles         4,49G  10,9G  4,49G  /rpool/GENTOO/distfiles
rpool/GENTOO/opt               1,93G  10,9G   984M  /rpool/GENTOO/opt
rpool/GENTOO/portage           1,40G  10,9G   897M  /rpool/GENTOO/portage
rpool/GENTOO/root              7,79G  10,9G  4,67G  legacy

It is needed to create layout that will not colide on both systems.

Boot partition looks like this:

root #ll /bootdir

total 33668
-rw-r--r--  1 root  wheel  2476465 Oct 30 00:02 System.map-3.10.7-gentoo-r1
-rw-r--r--  1 root  wheel    91809 Oct 30 00:02 config-3.10.7-gentoo-r1
drwxr-xr-x  2 root  wheel     1024 Nov 27 22:27 grub/
-rw-r--r--  1 root  wheel  2950360 Nov 27 17:57 initramfs.xz
drwx------  2 root  wheel    12288 Apr 30  2011 lost+found/
-rw-r--r--  1 root  wheel  3403920 Oct 30 00:02 vmlinuz-3.10.7-gentoo-r1

Create ZFS datasets

First we need to create zfs layout for freebsd:

zfs create rpool/FREEBSD
zfs create rpool/FREEBSD/ports
zfs create rpool/FREEBSD/ports/distfiles
zfs create rpool/FREEBSD/ports/packages
zfs create rpool/FREEBSD/root

Set some optional variables:

zfs set atime=off rpool/FREEBSD
zfs set compression=lz4
zfs set compression=off rpool/FREEBSD/ports/distfiles
zfs set compression=off rpool/FREEBSD/ports/packages

Unpack FreeBSD tarballs

cd /rpool/FREEBSD/
wget ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/9.2-RELEASE/base.txz
wget ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/9.2-RELEASE/kernel.txz
wget ftp://ftp.freebsd.org/pub/FreeBSD/releases/amd64/9.2-RELEASE/ports.txz

cd root
xzcat ../base.txz | tar -xv
xzcat ../kernel.txz | tar -xv
ln -s /rpool/FREEBSD/ports usr/ports
xzcat ../ports.txz | tar -xv

Prepare boot directory

Copy boot contents to boot partition.

 mkdir /boot/boot
 cd /rpool/FREEBSD/root
 mv boot/* /boot/boot/
 mv boot bootdir
 ln -s bootdir/boot boot

Set new boot parition in new FreeBSD fstab.

 echo '/dev/ada0s1 /bootdir ext2fs rw 0 1' > etc/fstab

Setup FreeBSD files

These settings are needed to make FreeBSD boot. For other options check freebsd handbook.

echo 'zfs_load="YES"' > /boot/boot/loader.conf
echo 'vfs.root.mountfrom="zfs:rpool/FREEBSD/root"' >> /boot/boot/loader.conf
echo 'ext2fs_load="YES"' >> /boot/boot/loader.conf

Set some variables to rc.conf

echo 'hostname="xxx"' > etc/rc.conf

Setup grub

Since I use grub-legacy, this apply to it. But not much different in grub-2.

Set menu entry in grub:

title FreeBSD
root (hd0,0)
kernel /boot/loader

Finish

Now if everything will goes well, you can reboot to new FreeBSD installation. For other setups consult freebsd handbook/wiki/forums.