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Converting a running physical machine into a KVM virtual machine

Posted on    7 mins read

Let’s assume you have a physical machine running a Linux system, and you would like to convert this system into a virtual KVM/QEMU machine, keeping everything as close to the original as possible. What follows is my approach.

The first thing we need is a raw image file which mirrors the exact layout of the physical hard drive in our physical server.

In our example scenario, the physical box has one hard drive at /dev/sda with a /boot Partition on /dev/sda2 and a physical LVM volume on /dev/sda3. This LVM volume houses a volume group with two logical volumes, one of them housing the root partition /, and the other one being unused. Also, /dev/sda1 is unused. Grub is installed into the Master Boot Record.

(While this setup may sound like it doesn’t make too much sense, trust me that I encountered this very setup the other day. The good news is that it’s an excellent example case because it is quite complicated, which gives me the chance to explain a lot of different concepts and solutions in detail.)

We need to recreate this hard drive in the virtual world. If we could stop the server, this would actually be quite simple: shutdown the machine, boot from a linux rescue cd, and dd every single byte from /dev/sda into a raw image file. This might even work when done from a running system where the disk is mounted, at least to a certain degree. But if you want to learn about all the little details that make up hard drive partitions and their file system, then continue reading.

Let’s look at the layout of the physical disk with fdisk -ul /dev/sda:

Disk /dev/sda: 299.4 GB, 299439751168 bytes
255 heads, 63 sectors/track, 36404 cylinders, total 584843264 sectors
Units = sectors of 1 * 512 = 512 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *          63     1012094      506016    b  W95 FAT32
/dev/sda2         1012095     1220939      104422+  83  Linux
/dev/sda3         1220940   584830259   291804660   8e  Linux LVM

Next, we need to switch to our KVM host server and create a raw image file that is exactly the same size as the physical hard drive (which, according to the first line of fdisk’s output, is 299.4 GB, or 299439751168 bytes):

# qemu-img create image.raw 299439751168

We could now re-create the partitioning scheme of the physical disk on the image by hand, but there is a simple shortcut: we only need to write the first 512 bytes of the physical disk into the first 512 bytes of the image. That’s the Master Boot Record (MBR) where all partitioning information resides.

dd is the tool of choice for reading and writing raw bytes. We use the following to read the MBR from our physical disk:

# dd if=/dev/sda of=./mbr.bin bs=512 count=1

It will write exactly one 512 byte block into a file called mbr.bin. Transfer this file to your KVM host, then write its content into the image file:

# dd if=./mbr.bin of=./image.raw bs=512 count=1 conv=notrunc

This writes exactly one block of 512 bytes into the image, and does not truncate the rest of the image.

Now run

# fdisk -l image.raw

to verify that the image file now has a partition layout which mirrors that of the physical disk:

    Device Boot      Start         End      Blocks   Id  System
image.raw1   *          63     1012094      506016    b  W95 FAT32
image.raw2         1012095     1220939      104422+  83  Linux
image.raw3         1220940   584830259   291804660   8e  Linux LVM

Now we can start to create filesystems on our imagefile’s partitions. But to create file systems, we need to address these partitions as devices. losetup comes to the rescue, because it allows us to present parts of a raw image file to the host system as loopback devices.

We need to create two loopback devices, one for the sda2 partition (/boot on our physical system), and one for the sda3 partition, which is a physical LVM volume. Afterwards, we will be able to use the sda2 loopback device directly – because of LVM, sda3 needs some extra care, as we will see.

Here is how we create a loopback device /dev/loop0 which points at the section of image.raw that makes up the sda2 partition:

# losetup /dev/loop0 image.raw -o 518192640 --sizelimit 106928128

You probably wonder where those insane numbers come from. It’s actually quite simple: The image file is, of course, just one continuous stream of bytes. A certain range of bytes within this stream represents the sda2 partition we just created on the image file. We don’t want the loopback device to point at the whole image file, but rather on the sda2 section only. And this section starts at byte 518192640 (the offset) and ends 106928128 bytes later (the sizelimit). How do we know? This is the calculation:

offset = partition start block * 512
sizelimit = (partition end block - partition start block) * 512

See the output of fdisk -l image.raw above for the partition start and end block numbers.

We now have a loopback device /dev/loop0 that looks and feels just like a real physical device – in this case, a hard disk partition. As this, it can be formatted:

# mkfs.ext3 /dev/loop0

Great, so now we have the /boot partition of our virtual server available, with the same layout as on our physical server. Let’s tackle the LVM volume on /dev/sda3 next.

What we need is the LVM header from our physical server’s disk. Again, dd is the tool of choice:

# dd if=/dev/sda3 of=lvmheader.bin bs=512 count=24

This writes the first 24 blocks of 512 bytes into the file lvmheader.bin. It’s the part if partition sda3 where the layout of the LVM setup is described. Just like the MBR, this needs to be transferred to our KVM host and must be written to the right place within our raw image file.

To do so, we will create another loopback device, /dev/loop1, which points at the byte section for the sda3 partition within our image file:

# losetup /dev/loop0 image.raw -o 625121280 --sizelimit 298807971328

The numbers were calculated accordingly, of course.

Now we can write the LVM header:

# dd if=lvmheader.bin of=/dev/loop1 bs=512 count=24 conv=notrunc

Afterwards, you can run

# pvs

which should display the newly found LVM volume group:

  PV         VG         Fmt  Attr PSize   PFree
  /dev/loop0 VolGroup00 lvm2 a-   278.28g    0


# lvm vgchange -ay

to activate it. Now, when running

# lvm lvs

its volumes should be listed like this:

  LV       VG         Attr   LSize   Origin Snap%  Move Log Copy%  Convert
  LogVol00 VolGroup00 -wi-a- 268.53g
  LogVol01 VolGroup00 -wi-a-   9.75g

At this point, the logical volume that will house the root partition can be accessed, and therefore we can format it:

# mkfs.ext3 /dev/mapper/VolGroup00-LogVol00

With this, we reached the point where we can mount the root and the /boot partition from the image on our KVM host:

# mount /dev/mapper/VolGroup00-LogVol00 /mnt
# mkdir /mnt/boot
# mount /dev/loop0 /mnt/boot

Next, we can copy over all the files from our physical server to the mounted image partitions on the KVM host. This could be done using rsync, for example:

# rsync -aAXvP / \
--delete \

The nice here is that you can transfer the files on-the-fly from the running system. Of course, at one point you need to make a “last sync” just before the virtual machine replaces the physical machine. However, you can transfer most of the data beforehand, and when the moment comes, you can shut down all services like databases, crond etc. and do the last sync in a relatively short time window.

Once this is done, we have a working, bootable KVM raw image. We could now unmount the partitions and import the image as a new virtual machine, like so:

# virt-install \
 -n mymachine \
 -r 512 \
 --os-variant rhel5.4 \
 --disk /var/lib/libvirt/images/image.raw,device=disk,bus=virtio,cache=none \
 --nonetworks \
 --graphics vnc,listen=,port=5910 \
 --import \

In case the VM does not boot, one reason could be that its initrd does not have the virtio drivers and thus cannot access the virtual drive. In this case, you must build a new initrd as follows:

If you still have the partitions mounted, bind the raw image file itself into the mounted filesystem, like so:

# mkdir /mnt/images
# mount --bind /var/lib/libvirt/images/image.raw /mnt/images

Now, chroot into your VM’s root partition:

# chroot /mnt

In there you must mount the /proc pseudo-filesystem, remove your current initrd image, and build a new one with the virtio drivers included:

# mount -t proc none /proc
# rm /boot/initrd.img
# mkinitrd --with virtio_pci --with virtio_blk /boot/initrd.img the.kernel.version
# umount /proc
# exit

Don’t forget to unmount everything afterwards:

# umount /mnt/boot
# umount /mnt/images
# umount /mnt
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