One of the beauties of hypervisor-based virtualization is in the commonality of its disk format. Disk formats are so common among virtualization platforms and versions that they've literally become standards themselves. As VMware's product lineup advanced, however, moving virtual machines from one VMware platform to another became less simple due to hardware incompatibilities. This article will review an important part of the VMware virtual machine creation process that helps prevent intra-VMware virtual machine migration difficulties later on.
Think back for a moment to the early days of VMware, when it seemed as if moving disk files between Workstation and ESX was a fantastically easy process. Dump any attached snapshots or REDO logs, copy the disk file from one location to the other using the vmkfstools command and you're off to the races. But as versions of Workstation, Server and ESX itself changed over the years, determining which formats worked with which versions became more difficult. What used to be a simple operation became a web of difficult-to-understand incompatibilities.
Fortunately, that is no longer the case. When installing the new VMware Server 2.0 RC2 and playing around with its interface, I came upon the Create Virtual Machine interface shown in Figure 1-1 below. There, when given the choice to select the given guest operating system, you are also given the option to view and select Product Capability. Although the ability to downgrade virtual machine capabilities at the point of creation to support their movement between VMware products has been around for a while, the screen below accomplishes the job in a way others have not. It spells out in a single glimpse the exact product versions that support each of the virtual hardware versions.
When creating a new virtual machine using VMware Server or VMware Workstation, you must take into consideration where you may want to move that machine in the future. As an example, if you ever intend this virtual machine to migrate to the ESX 3.0+ platform, the machine must be created with version 4 of its virtual hardware. Know that earlier versions of virtual hardware support fewer features, so this decision involves an obvious tradeoff.
You'll notice that VMware Server 2.0 creates virtual machines that are supported only on itself and VMware Workstation 6.5 by default. In fact, the only way to create virtual machines that are compatible with later released versions other VMware products is to revert the virtual machine's hardware all the way back to version 4.
For more detailed information on hardware version numbers, their capabilities, and the ease of moving between platforms, VMware keeps a document called the Virtual Machine Mobility Planning Guide. This document provides specific information about the difficulty associated with moving machines between platforms and among the various generations of VMware hardware. For non-trivial migrations, instructions are provided that assist you with getting your VMs moved from one location to another.
The moral of the story is that although the disk files may be appear to be the same across platforms and products, there are subtle differences. If you're a shop that leverages more than one VMware product and version, plan accordingly.
ABOUT THE AUTHOR: Greg Shields, MCSE, is a co-founder of Concentrated Technology with nearly 15 years of IT architecture and enterprise administration experience. He is an IT trainer and speaker on IT topics such as Microsoft administration, systems management and monitoring, and virtualization. His recent book, Windows Server 2008: What's New/What's Changed, is available from SAPIEN Press.