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Converting an existing Linux installation to ZFS root

The basic procedure is outlined here. These instructions are designed for Debian but may be adapted for other systems.

If you use LVM, use /dev/mapper/* names with ZFS for best results.


This guide is for those familiar with Linux, ZFS, and how Linux boots. If this whole page looks daunting to you, please don't attempt.

A comment: Booting from ZFS with LVM is not well-tested and has some pitfalls, but it can work. LVM makes a convenient way to copy data, though, if there is enough space free in your VG.

Any pool that grub will need to read must not use any raidz; it must be simple or mirrored.

You will either need to have enough spare space on your system to keep your data, or some place to move it to, then you can reformat the partitions and move it back. One nice hint: you can dd the live rescue ISO onto a USB memory stick, then use cfdisk or parted to add a partition consuming the spare space at the end, and use that partition to hold data. (Note: re-writing the ISO will erase the partition!)

Another hint: you can create your zpool on a new device temporarily, copy your data to it, unmount the source, then zpool attach the source partition as a mirror. Use zpool status to watch the copy progress, then zpool detach the temporary device when it's done. Just make sure that the temporary device is no larger than the place where you want it to go in the end. (It is fine if it is smaller, as long as it's big enough to hold your data.)


On the target system, install the ZFS packages using the zfsonlinux.org instructions, which are:

$ su -
# wget http://archive.zfsonlinux.org/debian/pool/main/z/zfsonlinux/zfsonlinux_2%7Ewheezy_all.deb
# dpkg -i zfsonlinux_2~wheezy_all.deb
# apt-get update
# apt-get install debian-zfs

Then, also install the ZFS versions of grub and the initramfs support:

apt-get install grub-pc zfs-initramfs

Make sure your grub has ZFS support:

dpkg -s grub-pc | grep Version

You should see "zfs" in that output string.

Set ZFS arc max

Create a file /etc/modprobe.d/local-zfs.conf and add:

options zfs zfs_arc_max=536870912

for a 512MB ZFS cache. This may need to be tweaked up or down on your system. See things like the ZFS Evil Tuning Guide or other references for guidance.

If you have already loaded the zfs module see lsmod | grep zfs, you may need to update-initramfs -u and then reboot for this to take effect.

zfs mounting workaround (for separate /usr, /var)

Bug reference: pkg-zfs #101

If you have /usr, /var, /home, etc. in separate zfs filesystems, the default zfs mount -a script runs too late in the boot process for most system scripts. To fix it, edit /etc/insserv.conf, and at the end of the $local_fs line, add zfs-mount (without a plus).

Also, edit /etc/init.d/zfs-mount and find three lines near the top, changing them like this:

# Required-Start:
# Required-Stop:
# Default-Start: S

Additional workaround for cryptdisks

If you have a zpool atop dm-crypt, you will also need to edit a few more files.


At the top, set

# Required-Start: cryptdisks-early

and before zfs mount -a, add:

zpool import -a

In mountall-bootclean.sh, set:

# Required-Start: mountall zfs-mount

Activating init.d changes

Then run:

insserv -v -d zfs-mount

If rpool is on LVM: initramfs bug workaround

Bug reference: pkg-zfs #102

If your rpool is on LVM, save this as /usr/share/initramfs-tools/scripts/local-top/jgoerzenactivatevg:

Syntax highlighting not supported for 'shell', see HelpOnParsers.
   1 #!/bin/sh
   3 # Workaround to make sure LVM is activated for ZFS
   4 # from http://wiki.complete.org/ConvertingToZFS by John Goerzen
   6 PREREQ="mdadm mdrun multipath"
   8 prereqs()
   9 {
  10         echo "$PREREQ"
  11 }
  13 case $1 in
  14 # get pre-requisites
  15 prereqs)
  16         prereqs
  17         exit 0
  18         ;;
  19 esac
  21 # source for log_*_msg() functions, see LP: #272301
  22 . /scripts/functions
  24 #
  25 # Helper functions
  26 #
  27 message()
  28 {
  29         if [ -x /bin/plymouth ] && plymouth --ping; then
  30                 plymouth message --text="$@"
  31         else
  32                 echo "$@" >&2
  33         fi
  34         return 0
  35 }
  37 udev_settle()
  38 {
  39         # Wait for udev to be ready, see https://launchpad.net/bugs/85640
  40         if [ -x /sbin/udevadm ]; then
  41                 /sbin/udevadm settle --timeout=30
  42         elif [ -x /sbin/udevsettle ]; then
  43                 /sbin/udevsettle --timeout=30
  44         fi
  45         return 0
  46 }
  49 activate_vg()
  50 {
  51         # Sanity checks
  52         if [ ! -x /sbin/lvm ]; then
  53                 message "jgoerzenactivatevg: lvm is not available"
  54                 return 1
  55         fi
  57         # Detect and activate available volume groups
  58         /sbin/lvm vgscan
  59         /sbin/lvm vgchange -a y --sysinit
  60         return $?
  61 }
  63 udev_settle
  64 activate_vg
  66 exit 0

Create pools and filesystems

Create your zpools and ZFS filesystems.

For instance:

zpool create rpool /dev/whatever
zfs create rpool/hostname-1
zfs create rpool/hostname-1/ROOT

If you want a separate /usr, /var, and /home, you might also:

zfs create rpool/hostname-1/usr
zfs create rpool/hostname-1/var
zfs create rpool/hostname-1/home

For swap, you might do this:

zfs create rpool/swap -V 1G -b 4K
mkswap -f /dev/rpool/swap

The -V gives the size of the swap, and the -b the blocksize. Per the zfsonlinux.org FAQ, the blocksize of swap should match the system's pagesize, and on amd64, that's 4K.

I usually recommend disabling atime on my systems, so:

zfs set atime=off rpool

Configure ZFS default

Edit /etc/default/zfs and set ZFS_MOUNT='yes'

Initial Copy

Now, you can prepare an initial run of populating the target filesystem with rsync. Be careful with how you do this!

rsync -avxHAXS --delete / /rpool/hostname-1/ROOT

For additional /usr, /var, etc:

rsync -avxHAXS --delete /usr/ /rpool/hostname-1/usr
rsync -avxHAXS --delete /var/ /rpool/hostname-1/var
rsync -avxHAXS --delete /home/ /rpool/hostname-1/home

The trailing slash after /usr/ is important.

Prepare for Reboot

Now, it is time to prepare the system for reboot.

First, we need to edit /etc/fstab. I'd start by saving it off:

cd /etc
cp fstab fstab.old

Now, you'll probably comment out everything that was converted to ZFS. Then add:

rpool/hostname-1/ROOT   /               zfs             defaults         0 0
/dev/rpool/swap         none            swap            sw              0 0

You do not need to list /usr, /var, etc. here since they will be auto-mounted by zfs.

Now, we need to configure the mountpoint for root in zfs. First, we have to unmount it so we can do this on a running system:

zfs umount -a
zfs set mountpoint=/ rpool/hostname-1/ROOT
zpool set bootfs=rpool/hostname-1/ROOT rpool

If you're using /usr, /var, and the like, also:

zfs set mountpoint=/usr rpool/hostname-1/usr
zfs set mountpoint=/var rpool/hostname-1/var
zfs set mountpoint=/home rpool/hostname-1/home

Finally, we unmount the zpool entirely:

zpool export rpool

Reboot to rescue disk

Now it's time to boot to the ZFS Rescue Disc to finish the installation.

Select live from the boot menu, and when you get the shell prompt, run sudo -s to become root.

Mount filesystems

You'll first mount the old filesystems on the rescue environment. We'll mount them under /tmp/old.

mkdir /tmp/old
mount /dev/blah /tmp/old
mount /dev/blah2 /tmp/old/usr

Then, you can import the pool under /tmp/new:

mkdir /tmp/new
zpool import -R /tmp/new rpool

Final Copy

This is the main reason we use the rescue disk: to get a good copy. A running system will have files in use, and also will have things mounted over places like /dev that mask what's there.

So, do a final rsync of all filesystems as before.

rsync -avxHAXS --delete /tmp/old/ /tmp/new
rsync -avxHAXS --delete /tmp/old/usr/ /tmp/new/usr
rsync -avxHAXS --delete /tmp/old/var/ /tmp/new/var
rsync -avxHAXS --delete /tmp/old/home/ /tmp/new/home

Most likely, there will be changes under / and /var but not /usr.

Prepare for booting

Now it's time to prepare for booting.

First, we set up the /tmp/new for chrooting:

mount -o bind /dev /tmp/new/dev
mount -o bind /sys /tmp/new/sys
mount -o bind /proc /tmp/new/proc

Now, we enter the new filesystem:

chroot /tmp/new

If you installed from the Debian Live CD, you'll have to remove the live tools:

dpkg --purge live-tools

Now, update the initramfs:

update-initramfs -k all -u

If you wish, you can make sure ZFS support was included:

mkdir /tmp/t
cd /tmp/t
zcat /boot/initrd.... | cpio -i
find . -iname '*zfs*'

You should see zfs.ko, /sbin/zfs, etc. listed.

Now, run:


and install grub:

grub-install /dev/whatever

Grub mirrored rpool workaround

Bug reference: zfsonlinux/grub #6

If you have a mirrored rpool, you may get an error at this point:

error: cannot find a GRUB drive for ...
....  Check your device.map.

If this happens, create /usr/local/bin/grub-probe, with these contents:

   1 #!/bin/bash
   3 /usr/sbin/grub-probe "$@" | head -1

and run:

grub-install --grub-probe=/usr/local/bin/grub-probe /dev/blah

Now, exit the chroot and umount everything:

umount /tmp/new/{proc,sys,dev}
zpool export rpool

Your system should now boot!


error: couldn't find a valid label (grub)

See http://comments.gmane.org/gmane.linux.file-systems.zfs.user/2485. This may occur if / is on ZFS but /boot is not. It may require fiddling with grub.cfg or the generating scripts. When I saw this, it just required me a press a key to continue booting.

/usr, /var, etc aren't mounting

See tips above.

See Also

Submitted Bugs

WikiCompleteOrg: ConvertingToZFS (last edited March 07, 2014 by JohnGoerzen)