Date: Thu, 23 Apr 2015 01:06:06 -0400 From: Eric McCorkle <eric@metricspace.net> To: freebsd-hackers@freebsd.org Subject: FreeBSD and Gentoo Linux on the same ZFS volume Message-ID: <55387DBE.7040706@metricspace.net>
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I had previously posted asking if anyone had ever gotten FreeBSD and a
linux distro to share the same ZFS volume. I have since accomplished
this setup with FreeBSD and Gentoo linux on a GPT disk with a single ZFS
volume. I'll write a quick-and-dirty description of how to do it here;
at some point in the future, I'll write a more in-depth wiki page about it.
== Setting up the Volume ==
Assume the pool's name is "data". I have the following datasets:
data/freebsd, mountpoint = legacy (root of FreeBSD)
data/freebsd/*, all mountpoints = legacy
data/home, mountpoint = /home (shared home dirs between linux and BSD)
data/linux, mountpoint = legacy (root of linux)
data/linux/*, all mountpoints = legacy
Note that only the datasets that are shared between the two OSes use the
ZFS mounting system. The ZFS code doesn't seem to support a "just mount
everything under my root" option (if it does, someone please point it
out). Thus, I have fstab entries for all the OS-specific datasets.
I also have a /linux directory under the FreeBSD root, and a /freebsd
directory under the linux root, at which the corresponding OS's
filesystems are mounted. Thus, each OS is accessible from the other. I
can see this being particularly useful to folks trying to port device
drivers.
Lastly, make sure to set data/freebsd as the boot dataset. Linux can
specify which dataset to use as root.
== Booting ==
Boot selection can be accomplished using GRUB. You can install GRUB
using the FreeBSD port. The MBR/bios solution should work for now; I'm
currently working on getting the EFI loader to work with ZFS, but it's
still a ways out.
Unfortunately, the port doesn't seem to be particularly well-documented.
The following procedure will set up an EFI-based GRUB with a linux and
a freebsd option (the freebsd option currently won't work with EFI and ZFS)
1) Install the sysutils/grub2-efi port
2) Edit /usr/local/etc/grub.d/40-custom file, add the following:
menuentry "Gentoo Linux" {
search.fs_label data ZFS_PART
linux ($ZFS_PART)/linux@/boot/kernel dozfs=force root=ZFS=data/linux
initrd ($ZFS_PART)/linux@/boot/initramfs
}
You'll also need to add a FreeBSD entry, as the script to detect FreeBSD
partitions doesn't seem to recognize ZFS.
3) Do "grub-mkconfig > /boot/grub.cfg" to generate the config file
4) Mount the EFI system partition under /mnt
5) Run "grub-install --efi-directory=/mnt --target=x86_64-efi
--modules=part_gpt". This will install the binary at
/mnt/efi/grub/grubx64.efi
6) Run "cp /mnt/efi/grub/grubx64.efi /mnt/efi/boot/bootx64.efi" to copy
GRUB into position to be run by the EFI loader.
It should be possible to rework this for legacy BIOS boot, which will
actually allow you to select the FreeBSD option and have it work.
== Installing FreeBSD ==
I currently have a legacy BIOS boot set up with a GPT disk, which boots
FreeBSD from the ZFS partition. There are guides for how to do this,
and they actually work. You can do a MBR/BSDlabel setup if your BIOS
can't do legacy boots with a GPT disk (Apple).
Be sure to update /etc/fstab too.
== Installing Gentoo Linux ==
The recent livedvd images have ZFS support, and will support the current
pool version. Older livedvds and other distros' installers sometimes
only support the older pool versions.
Once you get a livedvd with the right ZFS version, you can mount the
pool and follow the gentoo install guide for the most part. You'll have
to build a new kernel, install the ZFS ports, then build the initramfs,
which can be a bit tricky. Try to get the kernel/modules/initramfs
setup working enough to boot, then circle back and install the rest of
the OS.
Also, make sure you update /etc/fstab.
== Provisos ==
* I wouldn't recommend sharing swap space, as at least FreeBSD might
have saved a crash dump there (and I'm pretty sure linux swap has to be
prepared first)
* Sharing a L2ARC or intent log is also probably a bad idea. The
disjoint access patterns of the two OSes will mess up the locality
properties.
== Running ==
Once you've got everything set up, you should be able to boot either OS,
use a common /home, and then access the other OS from the /linux or
/freebsd directories.
As there isn't ZFS support in the EFI loader (yet!), I currently have to
switch to legacy BIOS boot mode to boot FreeBSD, and switch back to EFI
to boot gentoo (via GRUB).
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