Posts Tagged Hyper-V

Memory Overcommitment – What is it good for?

The statement, “there is no technical benefit to memory overcommitment” is usually met with universal scorn, righteous indignation and a healthy dose of boisterous laughter.  However, at second glance this statement is not quite so absurd. Overcommiting memory does not make your VMs faster, it doesn’t make them more highly available and it doesn’t make them more crash-resistant. So, what is memory overcommitment good for? The sole goal of memory overcommitment is to put more VMs per host.  This saves you money. Thus, the benefit that memory overcommitment provides is a financial benefit.

Are there other ways to attain this financial benefit?

Memory overcommitment is one of the main reasons people choose the ESX hypervisor.  If the goal of memory overcommitment is to save money and there are other ways to attain these cost savings on other hypervisors without utilizing memory overcommitment, does that change the hypervisor landscape at all?  Before delving into that question, let’s first see if there is a way to save as much money without using memory overcommitment.

One way around this I’ve heard suggested is to just increase the memory in your hosts.  So, if you had an ESX host with 10GB of memory and were 40% overcommitted then you could use XenServer or Hyper-V with the same amount of VMs but each host would have 14GB of memory.  This to me does not seem fair as you could also add 4GB more to your ESX host and achieve even more cost savings.  However, you can only add so much memory before becoming CPU-bound, right?  I’m not referring to CPU utilization but the amount of vCPU’s you can overcommit before running into high CPU Ready times.  Let’s use my earlier example.  You have 14, 1 CPU/1GB VMs on a 4CPU/10GB ESX host.  You want to put more VMs per host so you increase your host memory to 20GB.  You now try putting 28, 1CPU/1GB VMs on the host.  This is now twice the amount of vCPUs to the same amount of pCPUs and let’s say your CPU Ready times are around 5%-10%.  Adding more VMs to this host, regardless of how much more memory slots you have available, would adversely impact performance, so you have a ceiling of around 28 VMs per host.

Knowing this number, couldn’t you then size your hosts for 4CPU and 30GB of RAM on XenServer or Hyper-V and then be saving just as much money as ESX?  And this is only one way to recoup the financial benefits overcommitment provides you.  If you already have Windows or Citrix products you might already own these hypervisors from a license perspective and it might not save you money to go out and buy another hypervisor. Also, some hypervisors (like XenServer) are licensed by server count and not socket count (like ESX) so you could potentially save a lot of money by using these hypervisors.  In any of these cases, an in depth analysis of your specific environment will have to be done to insure you’re making the most cost effective decision.

Of course, memory overcommitment is not the only reason you choose a hypervisor.  There are many other factors that still have to be considered.  But given this discussion, is memory overcommitment one of these considerations?  I think once you realize what memory overcommitment is really good for, it becomes less of a factor in your overall decision making process.  Does this realization change the hypervisor landscape at all?  As I mentioned earlier, memory overcommitment is a major sell for ESX.  If you can attain the financial benefit of memory overcommitment without overcommiting memory then I think this does take a bite out of the VMware marketing machine.  That said, ESX is still the #1 hypervisor out there in my opinion and I would recommend it for the vast majority of workloads but not necessarily because of memory overcommitment.  There is room for other hypervisors in the datacenter and once people realize what memory overcommitment “is” and what it “isn’t” and really start analyzing the financial impact of their hypervisor choices I think you’ll see some of these other hypervisors grabbing more market share.

Thoughts?  Rebuttals?  Counterpoints?

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