ZFS as Root Filesystem

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This page is unofficial. ZFS as root filesystem is not supported under Funtoo Linux, mainly because it has limited benefit. ZFS is still supported but boot on a non-ZFS filesystem first. See ZFS.


This wiki article will show you how to install Funtoo on ZFS (rootfs).



ZFS is designed for 64-bit systems. We only recommend and support 64-bit platforms and installations!


ZFS v07.5 (latest in ports as of 18 Jan 2018) is compatible with kernel versions 2.6.32 - 4.14


The guide is under rewrite

It is recommended to give the entire disk to ZFS. As such, this guide will only show how to install ZFS on the whole disk, using legacy boot. Installing on UEFI requires a separate partition for /boot, formatted as FAT32, and is out of the scope of this guide, even though installation on UEFI is certainly possible. Also, this guide will not cover anything related to encryption (native encryption is available: https://github.com/zfsonlinux/zfs/commit/b52563034230b35f0562b6f40ad1a00f02bd9a05).

Downloading the ISO (With ZFS)

In order to install Funtoo on ZFS, you will need an environment, such as live media, that provides the ZFS tools. This guide will utilize the Ubuntu Desktop 16.04 (live) DVD for amd64.

  • Download from [1]

Creating a bootable USB from ISO (From a Linux Environment)

After you download the iso, you can do the following steps to create a bootable USB:


The size of the iso is approximately 1.5 GB.

Insert your blank USB media into a USB port. Then, inspect the kernel ring buffer with dmesg to identify the device name of your USB storage.

[  +5.533491] usb 6-2: new SuperSpeed USB device number 4 using xhci_hcd
[  +0.022995] usb 6-2: New USB device found, idVendor=1b1c, idProduct=1a0c
[  +0.000006] usb 6-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[  +0.000003] usb 6-2: Product: Voyager Mini 3.0
[  +0.000003] usb 6-2: Manufacturer: Corsair
[  +0.000002] usb 6-2: SerialNumber: 0123456789ABCDEF
[  +0.001095] usb-storage 6-2:1.0: USB Mass Storage device detected
[  +0.000080] scsi host15: usb-storage 6-2:1.0
[  +1.000772] scsi 15:0:0:0: Direct-Access     Corsair  Voyager Mini 3.0 PMAP PQ: 0 ANSI: 6
[  +0.000615] sd 15:0:0:0: Attached scsi generic sg6 type 0
[  +0.000110] sd 15:0:0:0: [sdg] 60566016 512-byte logical blocks: (31.0 GB/28.9 GiB)
[  +0.000209] sd 15:0:0:0: [sdg] Write Protect is off
[  +0.000004] sd 15:0:0:0: [sdg] Mode Sense: 2b 00 00 08
[  +0.000227] sd 15:0:0:0: [sdg] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[  +0.359532] sd 15:0:0:0: [sdg] Attached SCSI removable disk

In this example, [sdg] indicates that the device is /dev/sdg.

A quick and easy way to create a bootable USB is to write the ISO data to the USB device using dd.

root # dd if=/path/to/iso/ubuntu-16.04-desktop-amd64.iso of=/dev/sdg bs=4K

Once this has completed, remove and use this USB to boot the target system that will receive Funtoo Linux.

Booting the ISO

Using legacy (BIOS) boot mode, boot the ISO and allow Ubuntu to load the graphical environment. You will be presented with a "Welcome" dialog (titled Install (as superuser)). Select the option "Try Ubuntu".

Once the desktop has loaded, open the search bar by left-clicking on the top-left icon ("Search your computer"). Pressing Alt+F1 should also open this search bar. Search for and open the Terminal application.

In the terminal, issue the following commands to install and load the required ZFS module.

ubuntu@ubuntu:~$ sudo -i
root@ubuntu:~# apt-add-repository universe
root@ubuntu:~# apt update
root@ubuntu:~# apt install --yes debootstrap gdisk zfs-initramfs

Verify that the ZFS kernel module has loaded.

root@ubuntu:~#  dmesg | grep ZFS
[  377.595348] ZFS: Loaded module v0.6.5.6-0ubuntu10, ZFS pool version 5000, ZFS filesystem version 5

Drive Partitions

We are letting ZFS automatically partition the drive. This is ideal for our example using a single disk and legacy (BIOS) boot.

Creating of pool

Create a ZFS Storage Pool using a single whole disk.

root@ubuntu:~# zpool create -d -o feature@async_destroy=enabled -o feature@empty_bpobj=enabled -o feature@lz4_compress=enabled -o feature@spacemap_histogram=enabled -o feature@enabled_txg=enabled -o feature@extensible_dataset=enabled -o feature@bookmarks=enabled -f -o ashift=12 -o cachefile=/tmp/zpool.cache -O normalization=formD -O atime=off -O xattr=sa -O compression=lz4 -m none -R /mnt/funtoo rpool /dev/disk/by-id/foo

The options used here are

createUse zpool to create a ZFS Storage Pool.
-dRequired. Disables enabling all availabe zfs features. This is needed as the GRUB bootloader currently can't handle all advanced zfs features, not specifying this flag on pool creation (or running 'zpool upgrade' on the pool at a later point in time) will make the pool incompatible with GRUB.
(multiple) -o feature@<name>=enabledExplicitely enable all features currently supported by GRUB. See 'man zpool-features' for details on their effects.
-fForce the use of the selected disk.
-o ashift=12Alignment of the pool to underlying hard drive sectors. The recommended value is 12, which corresponds to 2^12 Bytes or 4 KiB. This value is typical for present-day HDD's. Can only be set once at pool creation.
-o cachefile=/tmp/zpool.cacheCreate a pool configuration cache and place it in /tmp. This will be required for our Funtoo install.
-O normalization=formDRecommended. Set the default Unicode (UTF-8) normalization for future filesystems (created within this pool) to 'formD'.
-O atime=offRecommended. As a default preference, set future filesystems (created within this pool) to not update file access time. Useful if we want to reduce writes to disk (e.g., Solid State Drives). Can cause problems for mailers and other software that rely on file access-time data.
-O xattr=saRecommended. As a default preference, set future filesystems (created within this pool) to store extended file attributes in a more efficient manner.
-O compression=lz4Recommended. As a default preference, set future filesystems (created within this pool) to be compressed using the lz4 algorithm. Useful in general as the runtime cost for compression is neglible on these days while on-disk space still costs money.
-m noneDo not set mountpoint for this storage pool. (This guide will address this later).
-R /mnt/funtooAlternate root directory. Essentially a temporary 'mount point' for our pool.
rpoolThe name of this ZFS Storage Pool. The pool name is irrelevant. rpool will be used throughout this guide.
/dev/disk/by-id/fooThe path to the physical disk. e.g. /dev/disk/by-id/ata-Samsung_SSD_840_EVO_120GB_123456789ABCDEF. Known in 'ZFS-speak' as a 'VDEV'.

Please note that the options 'ashift' and 'cachefile' are preceded with a lowercase '-o', while 'normalization' and 'atime' are preceded with an uppercase '-O'.

Without additional options, this will create our storage pool and enable all 'features' available under version The pool will be automatically mounted at the (temporary) location /mnt/funtoo.

To confirm the presence of our newly created pool:

root@ubuntu:~# zpool status
  pool: rpool
 state: ONLINE
  scan: none requested

	NAME                                             STATE     READ WRITE CKSUM
	rpool                                            ONLINE       0     0     0
	  ata-Samsung_SSD_840_EVO_120GB_123456789ABCDEF  ONLINE       0     0     0

errors: No known data errors

Create ZFS Datasets

Now, we will create one or more ZFS datasets within our storage pool. These will contain Funtoo Linux.

Create the Root file system (Required)

root #  zfs create -o mountpoint=none -o canmount=off rpool/ROOT
root #  zfs create -o mountpoint=/ rpool/ROOT/funtoo

Create optional Datasets

The following optional datasets are provided as examples. It is up to the user to create their own datasets.

root #  zfs create -o mountpoint=/home rpool/HOME
Build directory
root #  zfs create -o mountpoint=none -o canmount=off rpool/FUNTOO
root #  zfs create -o mountpoint=/var/tmp/portage -o compression=lz4 -o sync=disabled rpool/FUNTOO/build
Swap on ZFS

With some careful tuning a swap partition can be created on a ZFS 'volume' [2]. For a 2 GB swapfs:

root #  zfs create -V 2G -b $(getconf PAGESIZE) -o logbias=throughput -o sync=always -o primarycache=metadata rpool/swap
root #  mkswap /dev/zvol/rpool/swap
root #  swapon /dev/zvol/rpool/swap

Note that swap on ZFS has known stability issues. If the user has decided to use swap, please take following into account: Always use long /dev/zvol aliases in configuration files. Never use a short /dev/zdX device name.

To confirm the presence of the filesystems that we have created:

root@ubuntu:~# zfs list -t all
rpool               660K  19.3G    96K  none
rpool/HOME           96K  19.3G    96K  /mnt/funtoo/home
rpool/ROOT          192K  19.3G    96K  none
rpool/ROOT/funtoo    96K  19.3G    96K  /mnt/funtoo

Make the root filesystem bootable


Do not skip this!

When booting from ZFS, you must specify a boot device and a root file system within the pool that was identified by the boot device. By default, the dataset selected for booting is the one identified by the pool's bootfs property.

root # zpool set bootfs=rpool/ROOT/funtoo rpool

Installing Funtoo

Now that the initial ZFS pool and datasets have been created, we can unpack the stage 3 tarball and proceed with basic system installation. This part does not differ much from a regular Funtoo Linux installation (https://www.funtoo.org/Install#Installing_the_Stage_3_tarball). We recommend using funtoo-current stages for ZFS.

root # cd /mnt/funtoo
root # wget https://build.funtoo.org/next/x86-64bit/generic_64/stage3-latest.tar.xz

Extract the contents with the following command. If the user is using an arch-optimized stage 3 tarball, substitute in the actual filename.

root # cd /mnt/funtoo
root # tar xpf stage3-latest.tar.xz

Now, we need to create a chroot environment:

root # cd /mnt/funtoo
root # mount -t proc none proc
root # mount --rbind /sys sys
root # mount --rbind /dev dev

An important step is to copy the ZFS cache into the chroot. The ZFS cache was created when rpool was created with the -o cachefile=/tmp/zpool.cache flag.

root # mkdir -p /mnt/funtoo/etc/zfs
root # cp /tmp/zpool.cache /mnt/funtoo/etc/zfs/zpool.cache

You will also want to copy over resolv.conf in order to have proper resolution of Internet hostnames from inside the chroot:

root # cp /etc/resolv.conf /mnt/funtoo/etc/

We are now ready to chroot.

root # chroot /mnt/funtoo /bin/bash
root # export PS1="(chroot) $PS1"; cd

Configuring your system

Configure your system according to the main install guide. During startup, ZFS filesystems will be mounted without needing any entries in /etc/fstab. Comment out all entries in /etc/fstab except for partitions such as CD-ROMs, tmpfs, etc., if used.

If you created a swap volume earlier, add an appropriate entry to /etc/fstab.

root #  echo /dev/zvol/rpool/swap none swap defaults 0 0 >> /etc/fstab

Note that swap on ZFS is unstable. If you decide to use swap, please take following into account: always use long /dev/zvol aliases in configuration files. Never use a short /dev/zdX device name.

Next, update the portage tree:

root # ego sync
root # env-update
root # source /etc/profile

Installing ZFS userspace and bootloader

Installing the ZFS userspace tools and kernel modules

Install the ZFS packages and sync the portage tree, if required:

root # ego sync
root # emerge --ask sys-fs/zfs

Once it has successfully merged, add the following services to the boot runlevel of OpenRC:

root # rc-update add zfs-import boot
root # rc-update add zfs-mount boot

Add another two services to the default runlevel:

root # rc-update add zfs-share default
root # rc-update add zfs-zed default

Create a ZFS-friendly initramfs

The Funtoo stage3 includes a linux kernel and initramfs. The initramfs is designed to mount and start Funtoo Linux on a variety of file systems. The initramfs contained within the stage3 will not mount and start Funtoo in our ZFS storage pool. We must create an updated 'ZFS-friendly' initramfs.

Optional: Update to the latest No results:

root # emerge --oneshot sys-kernel/genkernel

Use genkernel to create an initramfs capable of mounting our ZFS Storage Pool via the --zfs switch. Adjust --makeopts according to the number of available threads:

root # genkernel initramfs --no-clean --no-mountboot --makeopts=-j4 --kernel-config=/usr/src/linux/.config --zfs
root ##g##* Funtoo Linux Genkernel; Version!g##
root ##g##*##!g## Running with options: initramfs --no-clean --no-mountboot --makeopts=-j4 --kernel-config=/usr/src/linux/.config --zfs

root ##g##*##!g## Linux Kernel 4.5.2-1 for x86_64...
root ##g##*##!g## .. with config file /usr/src/linux-debian-sources-4.5.2/.config
root ##g##*##!g## busybox: >> Using cache
root ##g##*##!g## initramfs: >> Initializing...
root ##g##*##!g##         >> Appending base_layout cpio data...
root ##g##*##!g##         >> Appending auxilary cpio data...
root ##g##*##!g##         >> Copying keymaps
root ##g##*##!g##         >> Appending busybox cpio data...
root ##g##*##!g##         >> Appending modules cpio data...
root ##g##*##!g##         >> Appending zfs cpio data...
cp: cannot stat ‘/etc/zfs/zdev.conf’: No such file or directory
root ##y##*##!y## Could not copy file /etc/zfs/zdev.conf for ZFS
root ##g##*##!g##         >> Appending blkid cpio data...
root ##g##*##!g##         >> Appending modprobed cpio data...
root ##g##*##!g##         >> Compressing cpio data (.xz)...

root ##y##*##!y## WARNING... WARNING... WARNING...
root ##y##*##!y## Additional kernel cmdline arguments that *may* be required to boot properly...
root ##y##*##!y## add "dozfs" for ZFS volume management support
root ##y##*##!y## add either "real_root=ZFS" (bootfs autodetection) or "real_root=ZFS=<dataset>" to boot from a ZFS dataset

root ##g##*##!g## Do NOT report kernel bugs as genkernel bugs unless your bug
root ##g##*##!g## is about the default genkernel configuration...
root ##g##*##!g## 
root ##g##*##!g## Make sure you have the latest ~arch genkernel before reporting bugs.

Confirm the presence of the new initramfs:

root # ls /boot/*genkernel*

Grub expects the initramfs filename to be of the form: initramfs-${KNAME}-${ARCH}-${KV}. This guide assumes the usage of debian-sources. Tailor it to your specific kernel name, platform, and kernel version. Forgetting to rename initramfs-genkernel to initramfs-debian-sources will render the system unbootable.

root # cd /boot
root # mv initramfs-genkernel-x86_64-4.5.2-1 initramfs-debian-sources-x86_64-4.5.2-1
root # ls /boot/*initramfs*

Installing GRUB 2

GRUB 2 must be built with support for ZFS Storage Pools on a single disk. This is achieved using the 'libzfs' USE flag.

root # echo "sys-boot/grub libzfs" >> /etc/portage/package.use

If you have defined GRUB_PLATFORMS in your /etc/portage/make.conf, please ensure that it includes 'pc': e.g. GRUB_PLATFORMS="efi-64 pc".

This is required for booting in BIOS (non-UEFI) mode, as described in this guide.

Now, install GRUB:

root # emerge grub

Configuring the Bootloader

When zpool created our storage pool (rpool), it created partitions under a GPT scheme. In order to boot Funtoo Linux on a GPT partion under legacy (BIOS) boot, sys-boot/grub requires a small partition, called a BIOS boot partition. By design, ZFS (zpool) left a very small unpartitioned space at the beginning of the disk. We will use sgdisk, which is part of sys-apps/gptfdisk, to format this free space into a BIOS boot partition.

root # sgdisk -a1 -n2:48:2047 -t2:EF02 -c2:"BIOS boot partition" /dev/disk/by-id/ata-Samsung_SSD_840_EVO_120GB_123456789ABCDEF

To avoid problems with GRUB, use partx to refresh the list of partitions that are 'seen' by the kernel. Do this for each drive (/dev/sda, /dev/sdb, etc.).

root # partx -u /dev/sda

A quick check to verify that GRUB 2 sees/supports ZFS:

root # touch /etc/mtab
root # grub-probe /

Installing GRUB2 to disk is as easy as:

root # grub-install /dev/disk/by-id/ata-Samsung_SSD_840_EVO_120GB_123456789ABCDEF
Installing for i386-pc platform.
Installation finished. No error reported.

Now, it's time for us to create grub's configuration file. First, we must edit a few GRUB 2 settings in /etc/default/grub:


The following is required to boot Funtoo Linux on ZFS!

Replace the line

   /etc/default/grub - before


   /etc/default/grub - after
GRUB_CMDLINE_LINUX="dozfs real_root=ZFS=rpool/ROOT/funtoo"

Now, create GRUB 2 configuration file:

root # grub-mkconfig -o /boot/grub/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/kernel-debian-sources-x86_64-4.5.2-1
Found initrd image: /boot/initramfs-genkernel-x86_64-4.5.2-1

If grub-mkconfig cannot find the initrd image, manually add initrd to the generated /boot/grub/grub.cfg, below the linux line:

   /boot/grub/grub.cfg - adding initrd below the linux line
echo    'Loading Linux x86_64-4.5.2-1 ...'
linux   /ROOT/funtoo@/boot/kernel-debian-sources-x86_64-4.5.2-1 root=ZFS=rpool/ROOT/funtoo ro dozfs real_root=ZFS
echo    'Loading initial ramdisk ...'
initrd  /ROOT/funtoo@/boot/initramfs-debian-sources-x86_64-4.5.2-1

A quick check to verify that the GRUB ZFS module is in place:

root # ls /boot/grub/*/zfs.mod

Final configuration

Configure your network according to the main installation guide.

Set the root password.

(chroot) # passwd

Exit the chroot and export your ZFS storage pool.

(chroot) # exit
root # umount -lR {dev,proc,sys}
root # cd /
root # zpool export rpool

Restart to boot into Funtoo Linux on a ZFS root!

After reboot


Take a snapshot of your Funtoo at installation.

root # zfs snapshot rpool/ROOT/funtoo@install

The use of snapshots, including sending snapshots as a method of backup, are not covered in this guide. See Further Reading.

ZFS Adjustable Replacement Cache (ARC) size

The Adjustable Replacement Cache (ARC) is a fundamental part of ZFS. Refer to this article by Aaron Toponce for details on how to set up an ARC.

Without configuration, ZFS will use up to 50% of your memory (RAM) for the ARC. It is possible to change this maximum. There are different ways to achieve this on both a temporary and persistent basis. One such way is to create and edit the file /etc/modprobe.d/zfs.conf, which affects the ZFS kernel module.

   /etc/modprobe.d/zfs.conf - set maximum ARC size to 4 GiB
options zfs zfs_arc_max=4294967296

where zfs_arc_max is set to a value in Bytes. After configuring this file, re-generate the initramfs. Rebooting will then apply this change.

To apply this change immediately without a reboot, issue the command:

root #  echo 4294967296 >> /sys/module/zfs/parameters/zfs_arc_max

Reference: https://wiki.gentoo.org/wiki/ZFS#ARC

After Kernel or ZFS updates

The default Funtoo Linux kernel (sys-kernel/debian-sources) does not automatically build a ZFS-capable initramfs. Similarly, the package sys-fs/zfs and its dependencies do not automatically build a ZFS-capable initramfs.

After each kernel update, you must recreate an initramfs. Similarly, sys-fs/zfs updates also require you to regenerate an initramfs. This is especially true for updates to sys-fs/zfs that introduce new storage pool features. Neglecting to update your initramfs will make your system unbootable.

First, rebuild sys-fs/zfs, sys-fs/zfs-kmod and sys-kernel/spl:

root # emerge --ask --verbose -1 zfs zfs-kmod spl

Then, follow the earlier instructions to create a new initramfs:

root # genkernel initramfs --no-clean --no-mountboot --makeopts=-j4 --kernel-config=/usr/src/linux/.config --zfs
root # cd /boot
root # mv initramfs-genkernel-x86_64-4.5.2-1 initramfs-debian-sources-x86_64-4.5.2-1

Finally, repeat the earlier instructions for generating /boot/grub/grub.cfg and manually adding the new initramfs as initrd to the generated /boot/grub/grub.cfg.

Further Reading

ZFS has many interesting features not covered by this guide.

Useful information and instructions can be found in the online reference manuals. See man zpool and man zfs.

Aaron Toponce's Zpool/ZFS Administration Guides - https://pthree.org/2012/12/04/zfs-administration-part-i-vdevs

ZFS on Linux - http://www.zfsonlinux.org

ZFS - Gentoo Wiki - https://wiki.gentoo.org/wiki/ZFS


Forgot to reset password?

You will need to chroot into your Funtoo on ZFS root installation if you forgot to set the root password. Repeat the earlier instructions to load the the live CD/USB, including the installation of ZFS kernel modules.

When the ZFS kernel modules are loaded, your existing ZFS Storage Pool (rpool) will be imported automatically. This will also result in an attempt to mount the various ZFS filesystems that you created. Mounting your root partition (/) will fail since this location is not empty; / is allocated to the live CD/USB distribution!

root # zpool status
  pool: rpool
 state: ONLINE
  scan: none requested

	NAME                                             STATE     READ WRITE CKSUM
	rpool                                            ONLINE       0     0     0
	  ata-Samsung_SSD_840_EVO_120GB_123456789ABCDEF  ONLINE       0     0     0

errors: No known data errors

To get around this issue, first, manually export the pool.

root # zpool export rpool
root # zpool list
no pools available

Now, import your storage pool using the following command.

root # zpool import -o cachefile=/tmp/zpool.cache -R /mnt/funtoo -d /dev/disk/by-id/ rpool

This will import rpool and place your Funtoo install at /mnt/funtoo. Follow the earlier set of instructions to chroot into your Funtoo Installation on ZFS root.

Will not mount on first reboot?

Follow the above instructions on "Forgot to reset password?" to import and mount your storage pool (rpool) and chroot back into your Funtoo environment.

Things to check:

  • Was the bootfs property of rpool set? Use zpool get bootfs rpool to check
  • Was /tmp/zpool.cache copied into /mnt/funtoo/etc/zfs/ prior to chroot and creating the initramfs?
  • Was genkernel initramfs run with the --zfs switch and --kernel-config pointing to the correct configuration file?
  • Was GRUB installed and configured correctly?

rpool is 'busy' on zpool export

This happens when swapon is used. Swap must be turned off to free the pool for export.

Unable to add universe repository in Ubuntu?

See http://askubuntu.com/questions/761592/unable-to-apt-get-dist-upgrade-on-a-persistent-ubuntu-16-04-usb