Virtualization Oversubscription - What’s so scary? (2 of 20) Fear and Misunderstanding

Carrying on from Wednesday I mentioned that ultimately there is a fear in these departments that oversubscription = poor performance. It’s considered to be a 1:1 relationship.  The reason for this is, to some extent, a misunderstanding of what oversubscription is.  It’s got the word ‘over’ in it then it must be bad.  Nothing in our department is ‘over’.  We’re all looking at the same word, they see something bad and I see an opportunity to save some money.  Correct me if you think I’m wrong, but saving money is typically thought to be a good thing.

Flying Navigation by Dead Reckoning

Avoiding oversubscription is a bit like navigating by dead reckoning.

• You know what you started with
• You know what you provisioned
• You know how much is left

In WW2 a bomb was considered to be on target if it was within 5 miles of the actual target and we only managed that with 1 in 5.  Dead reckoning isn’t very accurate on its own.  The situation is much more complex than that and the same remains true for people avoiding oversubscription.

Virtualization Used Capacity by Dead Reckoning

•        You know what you started with

•        You know what you provisioned

•        You know how much is left

This is not especially efficient. The essence of what these sites seem to be doing is this:

We start with a 5 Host cluster that has 120 Logical CPUs, and 180GB RAM.  We’re then going to issue no more than 96 vCPUs and 144GB RAM across the VMs.  This allows for a host to fail and we can still run everything.  We’ll also have great performance because VMs will get a CPU whenever they want it, because it’s theirs, and the same with memory.  All the memory a VM wants is real RAM.

I’m not going to deny that performance will be about as good as it can be, but it’s not going to be terribly efficient.  Chances are you could turn off 2 hosts and still see no impact in performance.  Who wouldn't like to reduce their ESX licence, and related power costs by 20%, while still having a spare host?

So what is oversubscription? I'll deal with this on Monday.

Phil Bell

Consultant