Managing virtual machines

Home Brew

Article from ADMIN 09/2012

By Oliver Frommel

With just a couple of command-line tools and DIY scripts, you can easily clone and manage your virtual machines. Dynamic DNS updates ensure that your virtual servers are immediately accessible on the network.

Virtual machines are a perfect choice if, for example, you need to test a system upgrade – something that always entails a certain amount of risk. So, when I wanted to see whether a full update from CentOS 5 to 6 would work – a procedure the release notes advise against – a virtual environment seemed like the prudent decision.

The first step is to create a copy of the current system, which should be as up to date as possible. Two options for this physical-to-virtual migration are to use special tools from the virt-v2v package [1] or to create an image over the wire with dd, but these aren't likely to give you consistent results with a running system. A cleaner approach would be to use an image backup program for Linux, such as Clonezilla [2] or Partimage [3], which means shutting down the system and booting a Live system.

To test the update, which doesn't require 100% synchronization, I decided to copy the CentOS installation to the server acting as the host using rsync.

This method should give you a trouble-free and fast option for updating the data after the first complete copy. You can then create a disk image for the virtual system with qemu-img or with guestfish [4], which will also create an ext3 filesystem, if so desired [5]:

$ guestfish -N fs:ext3

When you call Guestfish with the new image,

$ guestfish -a <Image>

you are taken to a shell where you first need to enter run to toggle the system to a ready state. Then, entering list-filesystems shows you the available partitions, and the built-in mount

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