Disc performance on VM using GPT and MBR

[SpLiX]

We're on our way to convert our old ESX 3.5 enviroent to vSphere and from Windows 2003 to 2008. The question is, if we should use GPT or MBR on our virtual discs. When we perform read/write tests, performance is better on a MBR disk compared to a GPT. Does anyone have an explanation to this? The discs we create is the same size, same block size and so on. The only difference is the disc type. GPT is supposed to be better in speed, allow bigger sizes then 2TB amongst many things. But so far our experience is that we should keep our VMs discs in the old format MBR.

I can't find any "best practise" or anything mentioned regarding how to optimize disc performance on a VM more then the basic disc alignment, nothing addresses the pros and cons for GPT vs MBR on a disc.

[bsti]

Here is a quick link that oulines the basic capabilities of the two: http://www.orthogonalthought.com/blog/index.php/2009/01/mbr-vs-gpt-comparison-windows-vista/

I don't know of a "best practices" doc per se (if someone does find one, that would be cool), but here is the way I'd think of it:

From everything I've read, there is no real performance gain or loss in going from GPT to MBR or vice versa. GPT is reported to perform better because it's automatically aligned whereas MBR needs to be aligned manually. Most people neglect this, thus it appears to perform not as well. The choice between GPT and MBR will more likely be based on need.

Basic things to consider:

- Obviously, if your partition needs to be > 2 TB, you need GPT

- If the partition needs to be booted from, it needs to be MBR (unless you have an Itanium system)

- If you need > 4 partitions, GPT

- If you have an older OS, MBR only

In your case, you'll probably be fine with MBR unless you anticipate growing the paritions over time to exceed 2 TB.

My two cents.