Ubuntu 18.04 Root on ZFS

Richard Laager edited this page Nov 16, 2018 · 34 revisions


  • This HOWTO uses a whole physical disk.
  • Do not use these instructions for dual-booting.
  • Backup your data. Any existing data will be lost.

System Requirements

  • Ubuntu 18.04.1 ("Bionic") Desktop CD (not any server images)
  • A drive which presents 512B logical sectors. Installing on a drive which presents 4KiB logical sectors (a “4Kn” drive) should work with UEFI partitioning, but this has not been tested.

Computers that have less than 2 GiB of memory run ZFS slowly. 4 GiB of memory is recommended for normal performance in basic workloads. If you wish to use deduplication, you will need massive amounts of RAM. Enabling deduplication is a permanent change that cannot be easily reverted.


If you need help, reach out to the community using the zfs-discuss mailing list or IRC at #zfsonlinux on freenode. If you have a bug report or feature request related to this HOWTO, please file a new issue and mention @rlaager.


This guide supports the two different Ubuntu encryption options: unencrypted and LUKS (full-disk encryption). ZFS native encryption has not yet been released.

Unencrypted does not encrypt anything, of course. All ZFS features are fully available. With no encryption happening, this option naturally has the best performance.

LUKS encrypts almost everything: the OS, swap, home directories, and anything else. The only unencrypted data is the bootloader, kernel, and initrd. The system cannot boot without the passphrase being entered at the console. All ZFS features are fully available. Performance is good, but LUKS sits underneath ZFS, so if multiple disks (mirror or raidz configurations) are used, the data has to be encrypted once per disk.

Step 1: Prepare The Install Environment

1.1 Boot the Ubuntu Live CD, select Try Ubuntu, and open a terminal (press Ctrl-Alt-T).

1.2 Setup and update the repositories:

$ sudo apt-add-repository universe
$ sudo apt update

1.3 Optional: Install and start the OpenSSH server in the Live CD environment:

If you have a second system, using SSH to access the target system can be convenient.

$ passwd
  There is no current password; hit enter at that prompt.
$ sudo apt install --yes openssh-server

Hint: You can find your IP address with ip addr show scope global | grep inet. Then, from your main machine, connect with ssh ubuntu@IP.

1.4 Become root:

$ sudo -i

1.5 Install ZFS in the Live CD environment:

# apt install --yes debootstrap gdisk zfs-initramfs

Step 2: Disk Formatting

2.1 If you are re-using a disk, clear it as necessary:

If the disk was previously used in an MD array, zero the superblock:
# apt install --yes mdadm
# mdadm --zero-superblock --force /dev/disk/by-id/scsi-SATA_disk1

Clear the partition table:
# sgdisk --zap-all /dev/disk/by-id/scsi-SATA_disk1

2.2 Partition your disk:

Run this if you need legacy (BIOS) booting:
# sgdisk -a1 -n2:34:2047  -t2:EF02 /dev/disk/by-id/scsi-SATA_disk1

Run this for UEFI booting (for use now or in the future):
# sgdisk     -n3:1M:+512M -t3:EF00 /dev/disk/by-id/scsi-SATA_disk1

Choose one of the following options:

2.2a Unencrypted:

# sgdisk     -n1:0:0      -t1:BF01 /dev/disk/by-id/scsi-SATA_disk1

2.2b LUKS:

# sgdisk     -n4:0:+512M  -t4:8300 /dev/disk/by-id/scsi-SATA_disk1
# sgdisk     -n1:0:0      -t1:8300 /dev/disk/by-id/scsi-SATA_disk1

Always use the long /dev/disk/by-id/* aliases with ZFS. Using the /dev/sd* device nodes directly can cause sporadic import failures, especially on systems that have more than one storage pool.


  • ls -la /dev/disk/by-id will list the aliases.
  • Are you doing this in a virtual machine? If your virtual disk is missing from /dev/disk/by-id, use /dev/vda if you are using KVM with virtio; otherwise, read the troubleshooting section.

2.3 Create the root pool:

Choose one of the following options:

2.3a Unencrypted:

# zpool create -o ashift=12 \
      -O atime=off -O canmount=off -O compression=lz4 -O normalization=formD \
      -O xattr=sa -O mountpoint=/ -R /mnt \
      rpool /dev/disk/by-id/scsi-SATA_disk1-part1

2.3b LUKS:

# cryptsetup luksFormat -c aes-xts-plain64 -s 256 -h sha256 \
# cryptsetup luksOpen /dev/disk/by-id/scsi-SATA_disk1-part1 luks1
# zpool create -o ashift=12 \
      -O atime=off -O canmount=off -O compression=lz4 -O normalization=formD \
      -O xattr=sa -O mountpoint=/ -R /mnt \
      rpool /dev/mapper/luks1
  • Setting atime=off completely disables atime updates. If you or one of your applications (e.g. a mail reader following an mbox file for local delivery) cares about atime updates, you almost certainly want relatime rather than full atime. In that case, replace atime=off with relatime=on. In steps below, when noatime is used in /etc/fstab, use relatime instead. If you only care about atime updates for particular filesystems, you can limit your relatime to only those filesystems; since properties are inherited, set atime=on to override the inherited atime=off wherever you set relatime=on. See RedHat's documentation for further information.
  • The use of ashift=12 is recommended here because many drives today have 4KiB (or larger) physical sectors, even though they present 512B logical sectors. Also, a future replacement drive may have 4KiB physical sectors (in which case ashift=12 is desirable) or 4KiB logical sectors (in which case ashift=12 is required).
  • Setting normalization=formD eliminates some corner cases relating to UTF-8 filename normalization. It also implies utf8only=on, which means that only UTF-8 filenames are allowed. If you care to support non-UTF-8 filenames, do not use this option. For a discussion of why requiring UTF-8 filenames may be a bad idea, see The problems with enforced UTF-8 only filenames.
  • Setting xattr=sa vastly improves the performance of extended attributes. Inside ZFS, extended attributes are used to implement POSIX ACLs (e.g. acltype=posixacl) discussed later. Extended attributes can also be used by user-space applications. They are used by some desktop GUI applications. They can be used by Samba to store Windows ACLs and DOS attributes; they are required for a Samba Active Directory domain controller. Note that xattr=sa is Linux-specific. If you move your xattr=sa pool to another OpenZFS implementation besides ZFS-on-Linux, extended attributes will not be readable. If this is important to you, omit the -O xattr=sa above.
  • Make sure to include the -part1 portion of the drive path. If you forget that, you are specifying the whole disk, which ZFS will then re-partition, and you will lose the bootloader partition(s).
  • For LUKS, the key size chosen is 256 bits. However, XTS mode requires two keys, so the LUKS key is split in half. Thus, -s 256 means AES-128, which is the LUKS and Ubuntu default.
  • Your passphrase will likely be the weakest link. Choose wisely. See section 5 of the cryptsetup FAQ for guidance.


  • The root pool does not have to be a single disk; it can have a mirror or raidz topology. In that case, repeat the partitioning commands for all the disks which will be part of the pool. Then, create the pool using zpool create ... rpool mirror /dev/disk/by-id/scsi-SATA_disk1-part1 /dev/disk/by-id/scsi-SATA_disk2-part1 (or replace mirror with raidz, raidz2, or raidz3 and list the partitions from additional disks).
  • The pool name is arbitrary. On systems that can automatically install to ZFS, the root pool is named rpool by default. If you work with multiple systems, it might be wise to use hostname, hostname0, or hostname-1 instead.

Step 3: System Installation

3.1 Create a filesystem dataset to act as a container:

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

On Solaris systems, the root filesystem is cloned and the suffix is incremented for major system changes through pkg image-update or beadm. Similar functionality for APT is possible but currently unimplemented. Even without such a tool, it can still be used for manually created clones.

3.2 Create a filesystem dataset for the root filesystem:

# zfs create -o canmount=noauto -o mountpoint=/ rpool/ROOT/ubuntu
# zfs mount rpool/ROOT/ubuntu

With ZFS, it is not normally necessary to use a mount command (either mount or zfs mount). This situation is an exception because of canmount=noauto.

3.3 Create datasets:

# zfs create                 -o setuid=off              rpool/home
# zfs create -o mountpoint=/root                        rpool/home/root
# zfs create -o canmount=off -o setuid=off  -o exec=off rpool/var
# zfs create -o com.sun:auto-snapshot=false             rpool/var/cache
# zfs create -o acltype=posixacl -o xattr=sa            rpool/var/log
# zfs create                                            rpool/var/spool
# zfs create -o com.sun:auto-snapshot=false -o exec=on  rpool/var/tmp

If you use /srv on this system:
# zfs create                                            rpool/srv

If this system will have games installed:
# zfs create                                            rpool/var/games

If this system will store local email in /var/mail:
# zfs create                                            rpool/var/mail

If you will use Postfix, it requires exec=on for its chroot.  Choose:
# zfs inherit exec rpool/var
# zfs create -o exec=on rpool/var/spool/postfix

If this system will use NFS (locking):
# zfs create -o com.sun:auto-snapshot=false \
             -o mountpoint=/var/lib/nfs                 rpool/var/nfs

If you want a separate /tmp dataset (choose this now or tmpfs later):
# zfs create -o com.sun:auto-snapshot=false \
             -o setuid=off                              rpool/tmp
# chmod 1777 /mnt/tmp

The primary goal of this dataset layout is to separate the OS from user data. This allows the root filesystem to be rolled back without rolling back user data such as logs (in /var/log). This will be especially important if/when a beadm or similar utility is integrated. Since we are creating multiple datasets anyway, it is trivial to add some restrictions (for extra security) at the same time. The com.sun.auto-snapshot setting is used by some ZFS snapshot utilities to exclude transient data.

Properties are inherited. If you want to create (for example) rpool/var/lib you may need to set -o exec=on manually (some apps, like Postfix, will need it).

We enable POSIX ACLs on /var/log for journald. See the note above in the zpool create step about xattr=sa being Linux-specific. That said, even if you do not want xattr=sa for the whole pool, it is probably fine to use it for /var/log.

If you want ACL support on other filesystems, set -o acltype=posixacl on them. If you want ACL support on everything, you can set it on the whole pool: zfs set acltype=posixacl rpool

If you do nothing extra, /tmp will be stored as part of the root filesystem. Alternatively, you can create a separate dataset for /tmp, as shown above. This keeps the /tmp data out of snapshots of your root filesystem. It also allows you to set a quota on rpool/tmp, if you want to limit the maximum space used. Otherwise, you can use a tmpfs (RAM filesystem) later.

3.4 For LUKS installs only:

# mke2fs -t ext2 /dev/disk/by-id/scsi-SATA_disk1-part4
# mkdir /mnt/boot
# mount /dev/disk/by-id/scsi-SATA_disk1-part4 /mnt/boot

3.5 Install the minimal system:

# chmod 1777 /mnt/var/tmp
# debootstrap bionic /mnt
# zfs set devices=off rpool

The debootstrap command leaves the new system in an unconfigured state. An alternative to using debootstrap is to copy the entirety of a working system into the new ZFS root.

Step 4: System Configuration

4.1 Configure the hostname (change HOSTNAME to the desired hostname).

# echo HOSTNAME > /mnt/etc/hostname

# vi /mnt/etc/hosts
Add a line:       HOSTNAME
or if the system has a real name in DNS:       FQDN HOSTNAME

Hint: Use nano if you find vi confusing.

4.2 Configure the network interface:

Find the interface name:
# ip addr show

# vi /mnt/etc/netplan/NAME.yaml
  version: 2
      dhcp4: true

Customize this file if the system is not a DHCP client.

4.3 Configure the package sources:

# vi /mnt/etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu bionic main universe
deb-src http://archive.ubuntu.com/ubuntu bionic main universe

deb http://security.ubuntu.com/ubuntu bionic-security main universe
deb-src http://security.ubuntu.com/ubuntu bionic-security main universe

deb http://archive.ubuntu.com/ubuntu bionic-updates main universe
deb-src http://archive.ubuntu.com/ubuntu bionic-updates main universe

4.4 Bind the virtual filesystems from the LiveCD environment to the new system and chroot into it:

# mount --rbind /dev  /mnt/dev
# mount --rbind /proc /mnt/proc
# mount --rbind /sys  /mnt/sys
# chroot /mnt /bin/bash --login

Note: This is using --rbind, not --bind.

4.5 Configure a basic system environment:

# ln -s /proc/self/mounts /etc/mtab
# apt update

# dpkg-reconfigure locales

Even if you prefer a non-English system language, always ensure that en_US.UTF-8 is available.

# dpkg-reconfigure tzdata

If you prefer nano over vi, install it:
# apt install --yes nano

4.6 Install ZFS in the chroot environment for the new system:

# apt install --yes --no-install-recommends linux-image-generic
# apt install --yes zfs-initramfs

4.7 For LUKS installs only:

# echo UUID=$(blkid -s UUID -o value \
      /dev/disk/by-id/scsi-SATA_disk1-part4) \
      /boot ext2 noatime 0 2 >> /etc/fstab

# apt install --yes cryptsetup

# echo luks1 UUID=$(blkid -s UUID -o value \
      /dev/disk/by-id/scsi-SATA_disk1-part1) none \
      luks,discard,initramfs > /etc/crypttab

4.8 Install GRUB

Choose one of the following options:

4.8a Install GRUB for legacy (MBR) booting

# apt install --yes grub-pc

Install GRUB to the disk(s), not the partition(s).

4.8b Install GRUB for UEFI booting

# apt install dosfstools
# mkdosfs -F 32 -n EFI /dev/disk/by-id/scsi-SATA_disk1-part3
# mkdir /boot/efi
# echo PARTUUID=$(blkid -s PARTUUID -o value \
      /dev/disk/by-id/scsi-SATA_disk1-part3) \
      /boot/efi vfat noatime,nofail,x-systemd.device-timeout=1 0 1 >> /etc/fstab
# mount /boot/efi
# apt install --yes grub-efi-amd64

4.9 Setup system groups:

# addgroup --system lpadmin
# addgroup --system sambashare

4.10 Set a root password

# passwd

4.11 Fix filesystem mount ordering

Until ZFS gains a systemd mount generator, there are races between mounting filesystems and starting certain daemons. In practice, the issues (e.g. #5754) seem to be with certain filesystems in /var, specifically /var/log and /var/tmp. Setting these to use legacy mounting, and listing them in /etc/fstab makes systemd aware that these are separate mountpoints. In turn, rsyslog.service depends on var-log.mount by way of local-fs.target and services using the PrivateTmp feature of systemd automatically use After=var-tmp.mount.

# zfs set mountpoint=legacy rpool/var/log
# zfs set mountpoint=legacy rpool/var/tmp
# cat >> /etc/fstab << EOF
rpool/var/log /var/log zfs noatime,nodev,noexec,nosuid 0 0
rpool/var/tmp /var/tmp zfs noatime,nodev,nosuid 0 0

If you created a /tmp dataset, do the same for it:
# zfs set mountpoint=legacy rpool/tmp
# cat >> /etc/fstab << EOF
rpool/tmp /tmp zfs noatime,nodev,nosuid 0 0

4.12 Optional: Mount a tmpfs to /tmp

If you chose to create a /tmp dataset above, skip this step, as they are mutually exclusive choices. Otherwise, you can put /tmp on a tmpfs (RAM filesystem) by enabling the tmp.mount unit.

# cp /usr/share/systemd/tmp.mount /etc/systemd/system/
# systemctl enable tmp.mount

Step 5: GRUB Installation

5.1 Verify that the ZFS root filesystem is recognized:

# grub-probe /

5.2 Refresh the initrd files:

# update-initramfs -u -k all
update-initramfs: Generating /boot/initrd.img-4.15.0-12-generic

Note: When using LUKS, this will print "WARNING could not determine root device from /etc/fstab". This is because cryptsetup does not support ZFS.

5.3 Optional (but highly recommended): Make debugging GRUB easier:

# vi /etc/default/grub
Comment out: GRUB_TIMEOUT_STYLE=hidden
Remove quiet and splash from: GRUB_CMDLINE_LINUX_DEFAULT
Uncomment: GRUB_TERMINAL=console
Save and quit.

Later, once the system has rebooted twice and you are sure everything is working, you can undo these changes, if desired.

5.4 Update the boot configuration:

# update-grub
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.15.0-12-generic
Found initrd image: /boot/initrd.img-4.15.0-12-generic

5.5 Install the boot loader

5.5a For legacy (MBR) booting, install GRUB to the MBR:

# grub-install /dev/disk/by-id/scsi-SATA_disk1
Installing for i386-pc platform.
Installation finished. No error reported.

Do not reboot the computer until you get exactly that result message. Note that you are installing GRUB to the whole disk, not a partition.

If you are creating a mirror, repeat the grub-install command for each disk in the pool.

5.5b For UEFI booting, install GRUB:

# grub-install --target=x86_64-efi --efi-directory=/boot/efi \
      --bootloader-id=ubuntu --recheck --no-floppy

5.6 Verify that the ZFS module is installed:

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

Step 6: First Boot

6.1 Snapshot the initial installation:

# zfs snapshot rpool/ROOT/ubuntu@install

In the future, you will likely want to take snapshots before each upgrade, and remove old snapshots (including this one) at some point to save space.

6.2 Exit from the chroot environment back to the LiveCD environment:

# exit

6.3 Run these commands in the LiveCD environment to unmount all filesystems:

# mount | grep -v zfs | tac | awk '/\/mnt/ {print $3}' | xargs -i{} umount -lf {}
# zpool export rpool

6.4 Reboot:

# reboot

6.5 Wait for the newly installed system to boot normally. Login as root.

6.6 Create a user account:

# zfs create rpool/home/YOURUSERNAME
# cp -a /etc/skel/.[!.]* /home/YOURUSERNAME

6.7 Add your user account to the default set of groups for an administrator:

# usermod -a -G adm,cdrom,dip,lpadmin,plugdev,sambashare,sudo YOURUSERNAME

6.8 Mirror GRUB

If you installed to multiple disks, install GRUB on the additional disks:

6.8a For legacy (MBR) booting:

# dpkg-reconfigure grub-pc
Hit enter until you get to the device selection screen.
Select (using the space bar) all of the disks (not partitions) in your pool.

6.8b UEFI

# umount /boot/efi

For the second and subsequent disks (increment ubuntu-2 to -3, etc.):
# dd if=/dev/disk/by-id/scsi-SATA_disk1-part3 \
# efibootmgr -c -g -d /dev/disk/by-id/scsi-SATA_disk2 \
      -p 3 -L "ubuntu-2" -l '\EFI\Ubuntu\grubx64.efi'

# mount /boot/efi

Step 7: Configure Swap

7.1 Create a volume dataset (zvol) for use as a swap device:

# zfs create -V 4G -b $(getconf PAGESIZE) -o compression=zle \
      -o logbias=throughput -o sync=always \
      -o primarycache=metadata -o secondarycache=none \
      -o com.sun:auto-snapshot=false rpool/swap

You can adjust the size (the 4G part) to your needs.

The compression algorithm is set to zle because it is the cheapest available algorithm. As this guide recommends ashift=12 (4 kiB blocks on disk), the common case of a 4 kiB page size means that no compression algorithm can reduce I/O. The exception is all-zero pages, which are dropped by ZFS; but some form of compression has to be enabled to get this behavior.

7.2 Configure the swap device:

Caution: Always use long /dev/zvol aliases in configuration files. Never use a short /dev/zdX device name.

# mkswap -f /dev/zvol/rpool/swap
# echo /dev/zvol/rpool/swap none swap defaults 0 0 >> /etc/fstab
# echo RESUME=none > /etc/initramfs-tools/conf.d/resume

The RESUME=none is necessary to disable resuming from hibernation. This does not work, as the zvol is not present (because the pool has not yet been imported) at the time the resume script runs. If it is not disabled, the boot process hangs for 30 seconds waiting for the swap zvol to appear.

7.3 Enable the swap device:

# swapon -av

Step 8: Full Software Installation

8.1 Upgrade the minimal system:

# apt dist-upgrade --yes

8.2 Install a regular set of software:

Choose one of the following options:

8.2a Install a command-line environment only:

# apt install --yes ubuntu-standard

8.2b Install a full GUI environment:

# apt install --yes ubuntu-desktop

Hint: If you are installing a full GUI environment, you will likely want to manage your network with NetworkManager:

# rm /etc/netplan/NAME.yaml
# vi /etc/netplan/01-netcfg.yaml
  version: 2
  renderer: NetworkManager

8.3 Optional: Disable log compression:

As /var/log is already compressed by ZFS, logrotate’s compression is going to burn CPU and disk I/O for (in most cases) very little gain. Also, if you are making snapshots of /var/log, logrotate’s compression will actually waste space, as the uncompressed data will live on in the snapshot. You can edit the files in /etc/logrotate.d by hand to comment out compress, or use this loop (copy-and-paste highly recommended):

# for file in /etc/logrotate.d/* ; do
    if grep -Eq "(^|[^#y])compress" "$file" ; then
        sed -i -r "s/(^|[^#y])(compress)/\1#\2/" "$file"

8.4 Reboot:

# reboot

Step 9: Final Cleanup

9.1 Wait for the system to boot normally. Login using the account you created. Ensure the system (including networking) works normally.

9.2 Optional: Delete the snapshot of the initial installation:

$ sudo zfs destroy rpool/ROOT/ubuntu@install

9.3 Optional: Disable the root password

$ sudo usermod -p '*' root

9.4 Optional:

If you prefer the graphical boot process, you can re-enable it now. If you are using LUKS, it makes the prompt look nicer.

$ sudo vi /etc/default/grub
Uncomment: GRUB_TIMEOUT_STYLE=hidden
Add quiet and splash to: GRUB_CMDLINE_LINUX_DEFAULT
Comment out: GRUB_TERMINAL=console
Save and quit.

$ sudo update-grub


Rescuing using a Live CD

Go through Step 1: Prepare The Install Environment.

This will automatically import your pool. Export it and re-import it to get the mounts right:

# zpool export -a
# zpool import -N -R /mnt rpool
# zfs mount rpool/ROOT/ubuntu
# zfs mount -a

If needed, you can chroot into your installed environment:

# mount --rbind /dev  /mnt/dev
# mount --rbind /proc /mnt/proc
# mount --rbind /sys  /mnt/sys
# chroot /mnt /bin/bash --login

Do whatever you need to do to fix your system.

When done, cleanup:

# mount | grep -v zfs | tac | awk '/\/mnt/ {print $3}' | xargs -i{} umount -lf {}
# zpool export rpool
# reboot


Most problem reports for this tutorial involve mpt2sas hardware that does slow asynchronous drive initialization, like some IBM M1015 or OEM-branded cards that have been flashed to the reference LSI firmware.

The basic problem is that disks on these controllers are not visible to the Linux kernel until after the regular system is started, and ZoL does not hotplug pool members. See https://github.com/zfsonlinux/zfs/issues/330.

Most LSI cards are perfectly compatible with ZoL. If your card has this glitch, try setting ZFS_INITRD_PRE_MOUNTROOT_SLEEP=X in /etc/default/zfs. The system will wait X seconds for all drives to appear before importing the pool.


Systems that require the arcsas blob driver should add it to the /etc/initramfs-tools/modules file and run update-initramfs -u -k all.

Upgrade or downgrade the Areca driver if something like RIP: 0010:[<ffffffff8101b316>] [<ffffffff8101b316>] native_read_tsc+0x6/0x20 appears anywhere in kernel log. ZoL is unstable on systems that emit this error message.


  • Set disk.EnableUUID = "TRUE" in the vmx file or vsphere configuration. Doing this ensures that /dev/disk aliases are created in the guest.


Set a unique serial number on each virtual disk using libvirt or qemu (e.g. -drive if=none,id=disk1,file=disk1.qcow2,serial=1234567890).

To be able to use UEFI in guests (instead of only BIOS booting), run this on the host:

$ sudo apt install ovmf
$ sudo vi /etc/libvirt/qemu.conf
Uncomment these lines:
nvram = [
$ sudo service libvirt-bin restart
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