Clone and Resize KVM Virtual Machine

(Up-to-date source of this post.)

I needed to upgrade (from Squeeze to Wheezy) some important virtual servers. As I wanted a minimal impact of the upgrade, I chose this procedure:

1. Create identical copy of the server to upgrade
2. Upgrade the copy
3. Upgrade the server if everything worked ok with the copy

The servers to upgrade were virtual machines (VMs) running on KVM. I also discovered that some servers needed more space because their disks had filled up during upgrade. So disk resize was needed. The following steps did the task:

1) Copy the image (.qcow2) and the configuration (.xml) files to some other location. The image file should ideally be copied from a snapshot to avoid data inconsistencies a running machine could create.

2) Edit the following fields in the copied .xml file accordingly

name
uuid
source dev    # make sure you enter the copied image path!
mac address
source bridge # change the VLAN to avoid IP address conflicts

3) Boot the cloned VM and change the hostname and IP address by editing these files:

/etc/network/interfaces
/etc/hostname
/etc/hosts

4) Change back the VLAN and shutdown the cloned VM

5) Increase the disk size

# convert the qcow image to a plain raw file
qemu-img convert system.qcow -O raw system.raw
# create a dummy file (filled with zeros) of the size of extra space you want to add to your image (here 1GB)
dd if=/dev/zero of=zeros.raw bs=1024k count=1024
# add your extra space to your raw system image without fear
cat system.raw zeros.raw > big.raw
# finally convert back your raw image to a qcow file not to waste space
qemu-img convert big.raw -O qcow growed-system.qcow

6) Boot the cloned VM and using cfdisk delete the old small partition and create a new one with the free space

7) Increase the filesystem using:

e2fsck -f
resize2fs

Make sure the VM's image file (.qcow) has the correct access rights, otherwise your system might have disk related problems (I was bitten by this and got helped by my nice colleague).