P2V: Thick and Thin disk formats

When you convert a physical server into a virtual machine using VMware converter, it is only possible to create full sized 'thick' disks. Ok, so you can resize the volume from the original server during the conversion, but the .vmdk file that you end up with is stored in thick format. This might not be what you want. It usually isn't. Most people want sparse disks now that thin provisioned disks are fully supported in vSphere. (The current version of converter is 4.1.1).

This is a typical scenario. You have a physical server with a 200 gigabyte system volume (C drive) of which about 20 is used. So what to do? During the conversion process, you'll be able to choose an option where you can resize the virtual disk as it comes across from the physical. This is pretty cool because in the example, you'll choose something like 40 and save a whole load of wasted disk space. That's exactly the sort of benefits virtualisation brings, so winner!

But. The virtual disk file you'll end up with in this example will be precisely 40 gig in size. The data 'inside' it will be the 20 gig that's used, but the file itself will be created at full size. This is because converter can only create thick format disks. This is not a problem in itself. Until vSphere supported thick disks within the GUI, it was only the command-line fiends (like me) who would be able to create thin disks. It always felt a bit risky though.

Now that it's supported, you might want your disks in thin format. There has been some debate about the performance of thin disks. VMware have settled this, for now anyway, with a whitepaper on the subject. In summary, it claims that "The test results show that thin disks perform as well as thick disks, even under I/O-intensive workloads." So that's alright then. (Published 30th Nov 2009).

Remember, your P2V'd disks are in thick format, so it's time to convert them to thin format. There are a few ways to do this.

- Any time you clone, deploy a template, or move a .vmdk using Storage vMotion, you can simply choose the option in the GUI.

- Using the vmkfstools command line tool with the -d option. You can do this directly inside the Service Console of an ESX server and via the vCLI for ESXi. (You ARE using ESXi right? You should be...)

Either of these processes involves making a copy of the virtual disk file. The data movers have the ability to inspect and deflate the image. It does have some potential disk IO impact, and required double the disk space at least temporarily. So be careful when you do this stuff; best to make these changes out of hours of possible.

References:

1) VMware whitepaper on Thick and Thin disk performance. (PDF)
2) vCLI Command reference guide (PDF)
3) vmkfstools Unix man page (internal link)