Cohabiting FreeBSD and Gentoo Linux on a Common ZFS Volume

My Librem 15 arrived a while back.  I normally prefer FreeBSD for just about everything, but I need access to a Linux OS running on the Librem platform in order to help me port over some remaining device drivers (namely the BYD mouse and screen brightness).

In order to facilitate this, I’ve installed a setup I developed a while back: that of a dual-booted FreeBSD and Gentoo Linux system living on the same ZFS volume.  This article details how to achieve this setup.

Note that this article is based on the EFI bootloader.  If you insist on legacy BIOS boots, you’ll need to adapt the procedure.

Overview of the Scheme

The scheme is based on a somewhat atypical use of the ZFS filesystem (namely foregoing the mountpoint functionality for the OS datasets in favor of an fstab-based approach) combined with GRUB to achieve a dual-bootable OS

ZFS Overview

The ZFS system differs slightly from the “typical” ZFS setup on both FreeBSD and Linux.  Some datasets (namely the home directories) are shared between both operating systems, but the OS datasets differ in their mount-points depending on which OS we are using, and thus the ZFS-specific mountpoint functionality can’t be effectively used.

In this article, assume that the volume’s name is “data”.

The overall scheme looks something like this:

  • data/home is mounted to /home, with all of its child datasets using the ZFS mountpoint system
  • data/freebsd and its child datasets house the FreeBSD system, and all have their mountpoints set to legacy
  • data/gentoo and its child datasets house the Gentoo system, and have their mountpoints set to legacy as well

OS and GRUB Overview

Both OSes must utilize the /etc/fstab method for mounting most of their filesystems, since we cannot make use of the ZFS mountpoint functionality.  This requires a different fstab for each OS.  Note that the data/home dataset (as well as any other similar dataset) will be mounted using the ZFS mountpoint method, not fstab.

Additionally, both OSes have access to the other OS’ data through a special top-level directory (/freebsd on Gentoo, /gentoo on FreeBSD).

The GRUB bootloader can be used to provide a workable boot selection facility without any serious modification or configuration (other than knowing the magic words to type into the grub.cfg file!)

Setup Procedure

The setup procedure consists of the following steps:

  1. Use the FreeBSD installer to create the GPT and ZFS pool
  2. Install and configure FreeBSD, with the native FreeBSD boot loader
  3. Boot into FreeBSD, create the Gentoo Linux datasets, install GRUB
  4. Boot into the Gentoo Linux installer, install Gentoo
  5. Boot into Gentoo, finish any configuration tasks

Note that nothing stops you from reversing the procedure, installing Gentoo first, and using its tools.  I just find that GPT creation and GRUB installation go a lot more smoothly on FreeBSD.

Getting Ready

To perform the setup procedure, you’ll need installer memstick images for both OSes.  FreeBSD’s installer can be gotten here; Gentoo’s can be gotten here (use the livedvd ISO).  You’ll also need some means of internet access (of course).

Note that in the case of the Librem 15 or similar laptops which have no ethernet connector, you may need to adopt a slightly modified procedure of installing Gentoo’s wireless tools and wpa_supplicant during the main installation.

FreeBSD Installer

Boot into the FreeBSD installer, go through the boot menu, and select manual partitioning mode.  This will drop you into a shell and ask you to create your partitions and mount everything at /mnt.

Create Partitions and ZFS Pool

The first thing to do is use the gpart tool to create your partitions.  FreeBSD’s man pages are rather good, so you can use “man gpart” to get the guide to the tool.  My procedure on the Librem 15 looks like this:

gpart create -s gpt ada0
gpart create -s gpt ada1
gpart add -t efi -l efi-system -s 200M ada0
gpart add -t freebsd-zfs -l zfs-data ada0
gpart add -t linux-swap -l swap -s 96G ada0
gpart add -t freebsd-zfs -l zfs-data-log -s 16G ada0
gpart add -t freebsd-zfs -l zfs-data-cache ada0

Then create a ZFS pool with the new partitions, and format the EFI system partition with the DOS filesystem (seriously, why do we still use that thing?):

newfs_msdos /dev/ada0p1
zpool create -m legacy -o atime=off -o checksum=sha256 data /dev/ada0p2 log /dev/ada0p2 cache /dev/ada0p3

Note that we’ve turned off atime (which reduces disk write traffic considerably) and set the checksum algorithm to sha256.

The ada1 disk is a SSD I had installed.  If you don’t have an SSD, it doesn’t make any sense to have a log or a cache.  The 16GB intent log is way overkill, but it reduces the strain on the device.  Note that we set the root dataset’s mountpoint to “legacy”.

Note that linux has its own swap format, so we can’t share the swap device.

Create the ZFS Datasets

Now that you have a ZFS pool, the first thing you’ll need to do is create the datasets.   Start by creating the FreeBSD root and mounting it (note that it will inherit the “legacy” mountpoint from its parent):

zfs create -o compression=lz4 data/freebsd
mount -t zfs data/freebsd /mnt/

We need to create some mountpoint directories:

mkdir /mnt/home
mkdir /mnt/gentoo/
mkdir /mnt/tmp
mkdir /mnt/usr
mkdir /mnt/var

I use a fairly elaborate ZFS scheme, which applies different executable, setuid, and compression properties for certain directories.  This achieves a significant compression ratio, effectively increasing the size of my disks:

zfs create -o exec=on -o setuid=off -o compression=off data/freebsd/tmp
zfs create -o exec=on -o setuid=on -o compression=lz4 data/freebsd/usr
zfs create -o exec=off -o setuid=off -o compression=gzip data/freebsd/usr/include
zfs create -o exec=on -o setuid=off -o compression=lz4 data/freebsd/usr/lib
zfs create -o exec=on -o setuid=off -o compression=lz4 data/freebsd/usr/lib32
zfs create -o exec=on -o setuid=off -o compression=gzip data/freebsd/usr/libdata
zfs create -o exec=on -o setuid=on -o compression=lz4 data/freebsd/usr/local
zfs create -o exec=on -o setuid=off -o compression=gzip data/freebsd/usr/local/etc
zfs create -o exec=off -o setuid=off -o compression=gzip data/freebsd/usr/local/include
zfs create -o exec=on -o setuid=off -o compression=lz4 data/freebsd/usr/local/lib
zfs create -o exec=on -o setuid=off -o compression=lz4 data/freebsd/usr/local/lib32
zfs create -o exec=on -o setuid=off -o compression=gzip data=freebsd/usr/local/libdata
zfs create -o exec=on -o setuid=off -o compression=gzip data/freebsd/usr/local/share
zfs create -o exec=off -o setuid=off -o compression=off data/freebsd/usr/local/share/info
zfs create -o exec=off -o setuid=off -o compression=off data/freebsd/usr/local/share/man
zfs create -o exec=on setuid=on -o compression=lz4 data/freebsd/obj
zfs create -o exec=on -o setuid=on -o compression=lz4 data/freebsd/usr/ports
zfs create -o exec=off -o setuid=off -o compression=lz4 data/freebsd/usr/ports
zfs create -o exec=on -o setuid=off -o compression=gzip data/freebsd/usr/share
zfs create -o exec=off -o setuid=off -o compression=off data/freebsd/usr/share/info
zfs create -o exec=off -o setuid=off -o compression=off data/freebsd/usr/share/man
zfs create -o exec=off -o setuid=off -o compression=gzip data/freebsd/usr/src
zfs create -o exec=off -o setuid=off -o compression=lz4 data/freebsd/var
zfs create -o exec=off -o setuid=off -o compression=off data/freebsd/var/db
zfs create -o exec=off -o setuid=off -o compression=lz4 data/freebsd/var/db/pkg
zfs create -o exec=off -o setuid=off -o compression=gzip data/freebsd/var/log
zfs create -o exec=off -o setuid=off -o compression=off data/freebsd/var/empty
zfs create -o exec=off -o setuid=off -o compression=gzip data/freebsd/var/mail
zfs create -o exec=on -o setuid=off -o compression=off data/freebsd/var/tmp

Because FreeBSD is pretty strict about where certain files go, this scheme works pretty well.  You could of course continue to subdivide it up to your heart’s desire, creating more subdirectories in /usr/share and such.

For Gentoo, you’ll probably want a simpler scheme, as Linux tends to be much sloppier about the locations of things:

zfs create -o exec=on -o setuid=off -o compression=off data/gentoo/tmp
zfs create -o exec=on -o setuid=on -o compression=lz4 data/gentoo/usr
zfs create -o exec=off -o setuid=off -o compression=lz4 data/gentoo/var

A Gentoo master might be able to subdivide this up as I’ve done with FreeBSD.

The final task is to mount all these filesystems manually with the following command template:

mount -t zfs data/freebsd/<path> /mnt/<path>

This is necessary, as all the mount-points will be “legacy”.  I won’t waste space by showing all the commands here.

Install and Configure the FreeBSD system

Now type “exit”, which will drop you back into the FreeBSD installer, with everything mounted at /mnt/.  The remainder of the installation procedure is straightforward.  However, you’ll need to opt to go to a shell for two final configuration tasks.

Go to the shell, then enter into the new FreeBSD system

chroot /mnt

Create the fstab

Since we mount the vast majority of the ZFS datasets to different paths in each OS, we’ll need to create an /etc/fstab file for them.  The following fstab will mount all the datasets to the right locations:

data/freebsd/tmp /tmp zfs rw 0 0
data/freebsd/usr /usr zfs rw 0 0
data/freebsd/usr/include /usr/include zfs rw 0 0
data/gentoo/ /gentoo zfs rw 0 0
data/gentoo/tmp /gentoo/tmp zfs rw 0 0
proc /proc procfs rw 0 0

Note that I’ve left out all a number of the entries.  You’ll have to map each dataset to its proper path as shown above.

Install the FreeBSD Native Bootloader

We’ll need the FreeBSD bootloader to get into the system for the first time.  Install it to the with the following procedure:

mount -t msdosfs /dev/ada0p1 /mnt
mkdir /mnt/efi
mkdir /mnt/efi/BOOT
cp /boot/boot1.efi /mnt/efi/BOOT/BOOTX64.EFI

The last thing you’ll need to do is set the bootfs parameter on the zpool, so the FreeBSD bootloader will pick the right dataset:

zpool set -o bootfs=data/freebsd data

You may also need to set the bootme flag on the EFI system partition for some hardware:

gpart set -a bootme -i 1 ada0

Your system is now ready to boot into the OS directly.

FreeBSD Main OS

You should now be able to boot into FreeBSD directly.  You’ll need to connect to a network, which may involve wpa_supplicant configuration.

Before doing anything else, I usually pull down the latest sources and rebuild world and the kernel first.  This ensures my system is up-to-date.  There are plenty of guides on doing this, so I won’t waste the space describing how to do it here.

You’ll also need to obtain the ports collection.  Again, there are plenty of guides on doing this.

Installing GRUB

The grub-efi port will install a version of GRUB capable of booting an EFI system.  This port is much easier to use in my opinion than the Gentoo equivalent.  The port is installed as follows:

cd /usr/ports/sysutils/grub2-efi
make install clean

At this point, you’ll need to create a grub.cfg file.  The grub-mkconfig command gets a good start, but you’ll inevitably need to edit it.  You can also just use the following file directly (make sure it’s at /boot/grub/grub.cfg):

insmod part_gpt
insmod zfs

menuentry 'FreeBSD' --class freebsd --class bsd --class os {
  search.fs_label data ZFS_PART
  echo "Loading FreeBSD Kernel..."
  kfreebsd ($ZFS_PART)/freebsd/@/boot/kernel/kernel
  kfreebsd_loadenv ($ZFS_PART)/freebsd/@/boot/device.hints
  kfreebsd_module_elf ($ZFS_PART)/freebsd/@/boot/kernel/opensolaris.ko
  kfreebsd_module_elf ($ZFS_PART)/freebsd/@/boot/kernel/acl_nfs4.ko
  kfreebsd_module_elf ($ZFS_PART)/freebsd/@/boot/kernel/zfs.ko
  set kFreeBSD.vfs.root.mountfrom=zfs:data/freebsd
  set kFreeBSD.vfs.root.mountfrom.options=rw

menuentry 'Gentoo Linux' {
  search.fs_label data ZFS_PART
  linux ($ZFS_PART)/gentoo@/boot/kernel dozfs=force root=ZFS=data/gentoo
  initrd ($ZFS_PART)/gentoo@/boot/initramfs

Note that we’ve created an entry for Gentoo, though it doesn’t yet exist.  Last, you’ll need to mount your EFI system partition and install GRUB:

mount -t msdosfs /dev/ada0p1 /mnt
grub-install --efi-directory=/mnt --disk-module=efi

This will install the GRUB boot program to /efi/grub/grub.efi on the EFI system partition.  You’ll need to copy it into place.  However, I recommend making a backup of your FreeBSD native bootloader first!

cp /mnt/efi/BOOT/BOOTX64.EFI /mnt/efi/BOOT/BOOTX64.BAK

This will simplify the recovery for you if things go wrong.  Now, copy the GRUB boot loader into place:

cp /mnt/efi/grub/grub.efi /mnt/efi/BOOT/BOOTX64.EFI

You should test your GRUB bootloader once to make sure it works by rebooting the system and booting into FreeBSD.  Don’t try to boot into Gentoo yet, as nothing is there!

Gentoo Installer

Your next task is to install the Gentoo base system.  Gentoo installation is done manually via the command line.  A guide is provided by the Gentoo Handbook.  Note that because you’re using ZFS as a root filesystem, you’ll need to do things a bit differently, and you will have to use genkernel to install your kernel!

Mount the Filesystems

As with FreeBSD, you’ll need to mount the filesystems:

zpool import -f data
mount -t zfs data/gentoo /mnt/gentoo
mkdir /mnt/gentoo/tmp
mkdir /mnt/gentoo/usr
mkdir /mnt/gentoo/var
mount -t zfs data/gentoo/tmp /mnt/gentoo/tmp
mount -t zfs data/gentoo/usr /mnt/gentoo/usr
mount -t zfs data/gentoo/var /mnt/gentoo/var

Now follow the Gentoo install steps and everything should go smoothly.

Creating the fstab

As with the FreeBSD system, you’ll need to create an /etc/fstab file.  The file looks similar to the FreeBSD version, but with the gentoo filesystems mounted relative to root and the FreeBSD filesystems mounted relative to /freebsd:

data/freebsd/tmp /freebsd/tmp zfs rw 0 0
data/freebsd/usr /freebsd/usr zfs rw 0 0
data/freebsd/usr/include /freebsd/usr/include zfs rw 0 0
data/gentoo/tmp /tmp zfs rw 0 0
data/gentoo/usr /usr zfs rw 0 0

Again, I’ve left out the repetitive portions of the file.

Building the Kernel, ZFS Modules, and initramfs

As we are booting from a root ZFS filesystem, you’ll need to set up a kernel with ZFS support.  You can find a guide to doing this here (skip down to the “Configuring the Kernel” section and go from there).

Note that I’ve set up the GRUB installation to go after /boot/kernel and /boot/initramfs.  Gentoo by default installs its kernel to /boot/kernel-<version information>, and the same with initramfs.  You’ll need to create symlinks with the name /boot/kernel and /boot/initramfs, or else copy the files into the right place yourself.

Final Gentoo Installation

Before you reboot, you’ll need to make sure that you’re read.  Here is a checklist of things I forgot and had to reboot into the installer to do:

  • Set a root password so you can actually log in
  • Install the ports to use wireless
  • Don’t miss a volume in /etc/fstab (if you miss /var, portage will get very confused)

Boot into Gentoo

You should now be able to boot into Gentoo directly from GRUB.  Congratulations!  You now have a dual-boot, single-ZFS system!  The last thing you’ll want to do before you create an user home directories is create a ZFS dataset for /home.  In the Gentoo system, do the following:

rm /home || rmdir /home
rm /freebsd/home || rmdir /freebsd/home
mkdir /home
mkdir /freebsd/home
zfs create -o mountpoint=/home -o exec=on -o setuid=off -o compression=lz4 data/home

You may also want to create datasets for specific users’ home directories (or even subdirectories thereof).  Note that we’ve set the mountpoint to /home.  This will cause the ZFS mountpoint functionality to mount those datasets, so there’s no need to add an fstab entry.


I have found this setup to be quite powerful and flexible, especially for kernel hackers and driver developers.  The following are some of the benefits of the setup:

  • Ready access to a full Linux system, including kernel sources from the FreeBSD system.
  • Convenient switching between systems for experimentation purposes
  • Effective recovery mechanism if one system gets broken

There’s also some interesting room for work with the FreeBSD Linux emulation layer here.  Normally, the FreeBSD Linux emulation ports install a minimal set of Linux packages.  I don’t know the subsystem well enough to do it myself, but I imagine there’s serious potential in having a full Linux installation at your disposal.

Author: Eric McCorkle

Eric McCorkle is a computer scientist with a background in programming languages, concurrency, and systems.

6 thoughts on “Cohabiting FreeBSD and Gentoo Linux on a Common ZFS Volume”

  1. Hi, Eric!

    First and foremost I’d like to thanks for your effort documenting this process. 🙂

    My question is: where did you find satisfiable documentation about the grub loader?
    My previous similar attempts for dual/multibooting linugz and other OSes were doomed to failure at the point I had to look for grub documentations. They were either outdated or not detailed enough.

    Thx, Máté


    1. I didn’t, really… I arrived at this config after a lot of experimentation and adapting various setups from other places.

      In fact, I originally wasn’t using the kfreebsd functionality at all, but rather doing a chainloader with loader.efi. I modified the setup generated by the grub-mkconfig script (which doesn’t seem to work out of the box, actually!) to get the current kfreebsd setup. I think I originally worked out the Gentoo-on-ZFS configuration setting up a work machine, but I can’t remember where I got the information anymore.


      1. ah, thx anyways

        not related:

        >gpart add -t freebsd-zfs -l zfs-data-log -s 16G ada0
        >gpart add -t freebsd-zfs -l zfs-data-cache ada0

        ada0 isn’t supposed to be ada1 here?


  2. Eric,
    I’ve tried to follow your method but the FreeBSD installer (using the FreeBSD DVD image) is complaining that it needs to have the root mount point set “/”. The installer just quits at this point. Have you seen this error before?



    1. As I recall, I simply set it that way, ran the installer, then opened a shell afterwards and unset it. I probably forgot to mention that.

      In my scheme, you’d set mountpoint on data/freebsd to /. Be sure to unset it afterwards, otherwise gentoo will mount the FreeBSD partition over its own root.


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