ZFSBootMenu is a Dracut module that intends to provide Linux distributions with an experience similar to FreeBSD's bootloader. By taking advantage of ZFS features, it allows a user to have multiple "boot environments" (with different distros, for example), manipulate snapshots before booting, and, for the adventurous user, even bootstrap a system installation via zfs recv.
In broad strokes, it works as follows:
- Via GRUB, direct EFI booting, etc, boot a Linux kernel along with an initramfs containing ZFSBootMenu.
- Look for
zfsbootmenuin the kernel command line.- Optionally specify a default pool (if multiple are present).
- Find all healthy ZFS pools and import them.
- If a specific pool was set, look for the
bootfspool value. Prefer this boot environment.- If no pool was defined in the command line, use the
bootfsvalue on the first-found pool. - If a
bootfsvalue is defined, start a 10 second (by default) countdown to boot that environment with the highest versioned kernel found in/boot. - If no
bootfsvalue is defined, find every filesystem that mounts to/with a/bootdirectory, and find every kernel image. Prompt for boot environment selection via a fuzzy finder.- If needed, prompt for encryption passphrases.
- If no pool was defined in the command line, use the
- Once the countdown has been reached for the bootfs-selected environment, prompt for the encryption passphrase if needed.
- Mount the filesystem and find the highest versioned kernel in
/bootin the selected boot environment. - Load the selected kernel and initramfs with the kernel command line defined in the
org.zfsbootmenu:commandlineproperty (or, as a fallback,/etc/default/grub) into memory withkexec. - Unmount all ZFS filesystems.
- Boot the final kernel and initramfs.
At this point, you'll be booting into your usual OS-managed kernel and initramfs, along with any arguments needed to correctly boot your system.
This tool makes uses of the following additional software:
ZFSBootMenu has been tested successfully with Kernel 5.8.14, Dracut 050 and OpenZFS 2.0.0-rc4.
To ensure the boot menu can find your kernels, you'll need to ensure /boot resides on your ZFS filesystem. An example filesystem layout is as follows:
NAME USED AVAIL REFER MOUNTPOINT
zroot 278G 582G 96K none
zroot/ROOT 10.9G 582G 96K none
zroot/ROOT/void.2019.10.04 1.20M 582G 7.17G /
zroot/ROOT/void.2019.11.01 10.9G 582G 7.17G /
zroot/home 120G 582G 11.8G /home
There are two boot environments created, identified by mounting to /. The environment that this system will boot into is defined by the bootfs value set on the zroot zpool.
NAME PROPERTY VALUE SOURCE
zroot bootfs zroot/ROOT/void.2019.11.01 local
On start, ZFSBootMenu will find the highest versioned kernel in zroot/ROOT/void.2019.11.01/boot, confirm that a matching initramfs is present, and default to booting the OS with that.
Kernel command line arguments should be configured by setting the org.zfsbootmenu:commandline ZFS property for each boot environment. If the property is not defined for a boot environment or its parents, command line arguments will be taken from the GRUB_CMDLINE_LINUX_DEFAULT variable defined in the boot environment's /etc/default/grub file, if it exists and the variable is set. Do not set any root= or any other pool-related options in the kernel command line; these will be filled in automatically when a boot environment is selected.
For example, I have the following command line arguments set for my boot environment:
zfs.zfs_arc_max=8589934592 elevator=noop
Because ZFS properties are inherited by default, it is possible to set the org.zfsbootmenu:commandline property on a common parent to apply the same arguments to multiple environments. Setting the property locally on individual boot environments will override the common defaults.
ZFSBootMenu integrates nicely with an EFI system. There will be two key things to configure here.
- The mountpoint of the EFI partition and its contents.
- The mountpoint of the boot environment
/bootand its contents.
Each boot environment should have its /boot directory in the ZFS filesystem. Using the above example, zroot/ROOT/void.2019.11.01 would contain /boot with kernel/initramfs pairs:
# ls /boot
config-5.3.18_1
config-5.4.6_1
efi
initramfs-5.3.18_1.img
initramfs-5.4.6_1.img
System.map-5.3.18_1
System.map-5.4.6_1
vmlinuz-5.3.18_1
vmlinuz-5.4.6_1
Once /boot is contained in a boot environment, it is necessary to install the boot menu files. Typically, EFI partitions (ESP) are mounted to /boot/efi, and contain a number of sub-directories. In this example, /boot/efi/EFI/void holds the ZFSBootMenu kernel and initramfs.
# lsblk -f /dev/sda
NAME FSTYPE LABEL UUID FSAVAIL FSUSE% MOUNTPOINT
sdg
├─sda1 vfat AFC2-35EE 7.9G 1% /boot/efi
└─sda2 swap 412401b6-4aec-4452-a6bd-6fc20fbdc2a5 [SWAP]
# ls /boot/efi/EFI/void/
initramfs-0.7.4.img
initramfs-0.7.5.img
vmlinuz-0.7.4
vmlinuz-0.7.5
With this layout, you'll now need a way to boot the kernel and initramfs via EFI. This can be done via a manual entry set via efibootmgr, or it can be done with rEFInd.
If you do not generate the ZFSBootMenu initramfs locally, you'll need to identify the following additional details:
- Your system's hostid (
hostid). It's important that this command be executed as root, to ensure that it returns the correct value. - Your boot pool name, if you have multiple.
- The disk path and partition index of your EFI partition. (
/dev/sda, part 1)
efibootmgr --disk /dev/sda \
--part 1 \
--create \
--label "ZFSBootMenu" \
--loader /vmlinuz-0.7.5 \
--unicode 'root=zfsbootmenu:POOL=zroot ro initrd=\EFI\void\initramfs-0.7.5.img quiet spl_hostid=a8c0a2a8' \
--verbose
Take note to adjust root=zfsbootmenu:POOL=, spl_hostid=, --disk and --part to match your system configuration.
Each time ZFSBootMenu is updated, a new EFI entry will need to be manually added, unless you disable versioning in the ZFSBootMenu configuration.
rEFInd is considerably easier to install and manage. Refer to your distribution's packages for installation. Once rEFInd has been installed, you can create refind_linux.conf in the directory holding the ZFSBootMenu files (/boot/efi/EFI/void in our example):
"Boot Default BE" "ro quiet loglevel=0 timeout=0 root=zfsbootmenu:POOL= spl_hostid="
"Select BE" "ro quiet loglevel=0 timeout=-1 root=zfsbootmenu:POOL= spl_hostid="
As with the efibootmgr section, the root=zfsbootmenu:POOL= and spl_hostid= options need to be configured to match your environment.
This file will configure rEFInd to create two entries for each kernel and initrams pair it finds. The first will directly boot into the environment set via the bootfs pool property. The second will force ZFSBootMenu to display an environment / kernel / snapshot selection menu, allowing you to boot alternate environments, kernels and snapshots.
The zfsbootmenu(7) manual page describes command line options for ZFSBootMenu kernels in detail.
The zfsbootmenu(7) manual page describes ZFS properties interpreted by ZFSBootMenu.
bin/generate-zbm can be used to create an initramfs on your system. It ships with Void-specific defaults in etc/zfsbootmenu/config.yaml. To create an initramfs, the following additional tools/libraries will need to be available on your system:
- For inclusion in the initramfs:
- For running
bin/generate-zbm:
If you want to create a unified EFI executable (which bundles the kernel, initramfs and command line), you will also need:
- linuxx64.efi.stub (typically packaged with gummiboot or systemd-boot)
Your distribution should have packages for these already.
config.yaml is used to control the operation of generate-zbm.
In prior versions of ZFSBootMenu, an INI format was used for configuration. In general, migration to the new format is not automatic, but generate-zbm can perform the migration if your distribution package has not done it for you. To migrate an existing configuration, just run
generate-zbm --migrate [ini-config] [--config yaml-config]
By default, the output YAML will be written to /etc/zfsbootmenu/config.yaml; use the --config argument to customize the output location.
The argument [ini-config] to --migrate is optional. When it is not provided, generate-zbm will derive an input file by replacing the .yaml extension from the output file with a .ini extension.
If (and only if) generate-zbm is run without a --config option (i.e., it attempts to load the default /etc/zfsbootmenu/config.yaml) and the default configuration does not exist, generate-zbm will behave as if it had been passed the --migrate /etc/zfsbootmenu/config.ini option.
Whenever generate-zbm attempts to migrate configuraton files, it will exit with a zero exit code on successful conversion and a nonzero exit code if problems were encountered during the conversion. No boot images will be produced in the same invocation as a migration attempt.
ZFSBootMenu can import pools or filesystems with native encryption enabled. If your boot environments are not encrypted but, for example, /home is, you will not receive a decryption prompt during boot. To ensure that you can decrypt your pool to load the kernel and initramfs, you'll need to you have the filesystem parameters configured correctly.
zfs get all zroot | egrep '(encryption|keylocation|keyformat)'
zroot encryption aes-256-gcm -
zroot keylocation file:///etc/zfs/zroot.key local
zroot keyformat passphrase -
zroot encryptionroot zroot -
It's critical that keyformat is set to passphrase, otherwise you'll be unable to enter the correct value in the boot loader. ZFS on Linux currently supports only one key, but in a way which we can exploit: if you configure the keylocation value to a file on disk, put your passphrase in that, and then include that file into the FINAL initramfs (the OS-managed one), you won't receive a second password prompt on boot. You'll still receive a password prompt in the boot loader, since we can force a prompt for passphrase input.
For Dracut-based systems, this can be done by creating a /etc/dracut.conf.d/zol.conf file with the following contents:
install_items+=" /etc/zfs/zroot.key "
It's critical that you do not include this key file into the ZFSBootMenu initramfs, since that file exists on an unencrypted volume - leaving your pool essentially wide-open.