HOW-TO: Prep Work for Linux P2V (Physical to Virtual)

Virtualization is the technology trend that is "in" nowadays. It is debatable whether the money you save in hardware costs (and other variable costs) can equate or best the savings of whatever virtualization technology deployed or implemented. But that is beyond the point of this article.

Physical to virtual (otherwise known as P2V) is the process of converting a baremetal host to a virtual equivalent. For Linux hosts, there is only one way to do this. P2V is done from a Windows host that has the VMware Standalone Converter installed. The best way to do this without downtime is to do your homework prior to. Prep work is vital in a production environment.

The quickest way to avoid downtime is to choose to shutdown the physical machine and boot the virtual machine when conversion (or P2V) is done. This is via checking a tick box during the conversion process. However, in my experience the virtual machine doesn't always boot after the conversion. How then do we ensure that the virtual machine boots up after conversion?

One key strength of VMware is its virtual machines (and their corresponding drivers) are assigned or given a generic set of hardware. The hardware on the VMs are as generic as possible for maximum compatibility. This enables the VM to migrate (or vMotion) to compatible architectures with least impact or even no impact at all. The end-user will not even feel the vMotion occur or experience any glitch while the machines migrate from one ESXi host to the other. While a continuous ping is done it might drop one or two packets while a vMotion is going on but that really is not felt by the user most of the time.

Given the above, one way to ensure a successful P2V is that hardware specific limitations do not exist on the virtual machine (or VM). What then are hardware specific limitations? The MAC address is one. Virtual machines are automatically assigned a different MAC address than the one on the physical host. If the MAC address is hard-wired, then obviously the resulting Linux VM will not like it.

On Fedora flavors of Linux as well as its Community operating system counterpart, there are two locations where the MAC address is hard-wired or hard-coded. The file /etc/udev/rules.d/70-persistent-net.rules contain the hard-coded MAC address. As seen from the screenshot below, the host has two (2) network interface cards, and CentOS has detected them all and assigned them names eth0 and eth1. Their corresponding MAC addresses are also listed. This way the OS knows which NIC is assigned what name.

Sometimes, the files ifcfg-eth0, ifcfg-eth1 -- located in /etc/sysconfig/network-scripts -- also contain the hard-coded MAC addresses. This is another way for the OS to know the particular NIC has been assigned a corresponding device name. Below are entries for the file ifcfg-eth0.

On both of the above scenarios, you may choose to delete the lines containing the MAC addresses or place a hash (#) in the beggining of the line to make the OS ignore them.

Other times, the VM refuses to boot due to a missing module in the initrd file. And the solution could vary between scenarios. Still, it helps to do your homework prior to so you can minimize downtime and anticipate headaches that may come your way.


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