Moving a Linux root filesystem to LVM


Many linux users install the root filesystem on a standard partition. If this is the case with you, it may be possible to copy the root filesystem onto an LVM Logical-Volume by hand, and to modify the boot loader configuration file (eg: /boot/grub.conf for Grub) so that it now boots from the LVM volume. This procedure has been tested with Debian Lenny, and it’s likely to work for other distributions.

You will need enough space on your disk to have both the old root partition and the new LVM volume at the same time. If you don’t have enough space, it’s also possible to backup and restore your root filesystem instead of doing a copy. It should work if you know what you are doing. This is a dangerous operation, so please check that you have a recent backup first and don’t do this if you don’t fully understand this procedure. It is recommended that you read the previous pages about LVM before reading this one.

Checking the initramfs

Before you migrate your root filesystem to an LVM volume, you should check that the initramfs provided by your distribution supports LVM. On some distributions, the initramfs is configuration specific. For instance, on Fedora/Redhat, the initramfs is created automatically by a program called mkinitrd each time you install or update the kernel image. The problem is that this program only includes the disk drivers for the hardware on which it runs, and it only includes the LVM programs if you are using it. In that case you will have problems if you try to reuse this linux kernel image and this initramfs on hardware which needs different disk drivers and if you use LVM. The solution is to recreate the initramfs with LVM support before you reboot the new root filesystem.

With Other distributions such as Debian-Lenny, the initramfs includes all the disk drivers and it always includes the LVM programs so it should work. In that case you can just reuse the linux -kernel-image and the initramfs as they have already been installed in /boot.

As of version 09.10 the default installer in Ubuntu does not support LVM. You have to install Ubuntu on a normal partition first, run commands on this system either by booting on it or through chroot, install the lvm2 tools, update the initramfs and then copy the filesystem to an LVM volume:

  • Install Ubuntu on a normal partition (eg: /dev/sda1)
  • Boot on the new installed system or chroot to it from SystemRescueCd
  • Install the lvm2 tools: apt-get install lvm2
  • Regenerate the initramfs: update-initramfs -u -k all

Then the initramfs will have LVM support and it will work when you try to use it to boot Ubuntu from an LVM Logical-Volume.

Copying the root filesystem to a logical volume

  • Boot from a recent SystemRescueCd You can boot from the cdrom edition or a usb or from the network
  • Create an LVM Physical Volume if necessary If you have no LVM Physical-Volume, you have to find a disk or a partition for it. It will need to be big enough to store the new root filesystem
  • Create an LVM Volume-Group if necessary Create an LVM Volume-Group if you don’t have one already. In this example it will be called /dev/vgmain
  • Create an LVM Logical-Volume Create an LVM Logical-Volume which is big enough for the root filesystem. Let’s say it’s /dev/mapper/vgmain-debian
  • Make a new filesystem on the new Logical-Volume You can change the type of the filesystem if you are sure that the kernel will support it. If you are not sure which filesystems are supported by your distribution, you should keep the same one, it’s ext3 in general:
    mke2fs -j -L debian /dev/mapper/vgmain-debian
  • Mount both the old and the new filesystems
    mkdir -p /oldrootfs /newrootfs
    mount -r /dev/sda2 /oldrootfs
    mount /dev/mapper/vgmain-debian /newrootfs
  • Copy the contents of the root filesystem to the new volume rsync will be used to copy all the data because it know how to preserve all file attributes, including the extended-attributes which are required for selinux to work when it’s enabled. You can also use another tool such as tar if you know what you are doing. You can also make a copy at the block level if the new volume is at least as big as the old one but you may have to run resizefs if it’s bigger. Here is how to do the copy using rsync:
    rsync -axHAX /oldrootfs/ /newrootfs/
  • Update the entry for the root filesystem in the new fstab You have to edit /newrootfs/etc/fstab and change the entry related to the root filesystem so that the name of the device and the filesystem are correct.
  • Unmount the filesystems
    cd / ; umount /oldrootfs /newrootfs

Updating the boot loader

Now you have to mount your boot partition and edit the boot loader configuration so that it will know from where to boot. It’s recommended to preserve the existing boot entry, just in case there is a problem with the new root filesystem. Here is an example of a grub configuration file for Debian with these two entries. The important thing is the the root=xxx boot parameter.

default 0
timeout 10

title Debian-Linux-2.6.26-2-amd64 [new-lvm-rootfs]
        root (hd0,0)
        kernel /vmlinuz-2.6.26-2-amd64 root=/dev/mapper/vgmain-debian ro
        initrd /initrd.img-2.6.26-2-amd64

title Debian-Linux-2.6.26-2-amd64 [old-std-rootfs]
        root (hd0,0)
        kernel /vmlinuz-2.6.26-2-amd64 root=/dev/sda2 ro
        initrd /initrd.img-2.6.26-2-amd64

Now you should be able to reboot on the new root filesystem.

Manual (EN)
LVM Guide
Disk partitioning

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