James O'Neill's Blog

March 15, 2008

VMware – running hot enough to cook the figures

Filed under: Virtualization,Windows Server,Windows Server 2008 — jamesone111 @ 3:38 am

It’s quite amusing to see the VMware blogger (Citrix’s blog names him as Eric Horschmann) has come back with a screen full of algebra to try justify it the post I commented on yesterday

  VMware VI3 foundation VMware VI3 enterprise Free Hypervisor
2 way Server, 4GB Ram $6000 $6000 $6000
Windows Licences $5998 $5998 $5998
Virtualization licences $995 $5750 $0
Total Cost $12,993 $17,748 $11,998
VMs 14 (with 2x overcommit) 14 (with 2x overcommit) 7
Price per VM $928 $1268 $1714.

They look convincing, don’t they ?  He’s right only if you fixed the amount of RAM in the serverBut when was that ever a fixed point ? either you want to run 14 VMs on the box, or the budget per box is $17,748. Who ever yelled "Screw the budget, Screw the workload. Keep the memory constant !" ?

Needless to say if your product is adding cost, you want the other figures beefed up to keep the proportion due to you as small as possible. I said the cost of the box was suspiciously high. And would you run 0.5 GB VMs (was that why they chose an out of support OS) . If you were only running 1GB VMs you’d get 3 on the box and use the cheaper Enterprise edition of Windows rather than Datacenter. That would reduce the cost of the Microsoft system, perhaps to the point where it was as cheap to run two small Microsoft boxes as one larger VMware one. 

So lets try those figures again shall we ?  This time we’ll build a server to support 14 VMs. The only cha

  VMware VI3 foundation VMware VI3 enterprise Free Hypervisor
VMs 14 14 14
2 way Server, 4GB Ram $6000 $6000 $6000
4GB extra RAM Not needed
(with 2x overcommit)
Not needed
(with 2x overcommit)
Windows Licences $5998 $5998 $5998
Virtualization licences $995 $5750 $0
Total Cost $12,993 $17,748 $12,493
Price per VM $928 $1268 $892

Who’s cheapest now  ?

Now if the customer is prepared to $5,750 (plus support, training, and extra management tools) on VI3 enterprise… what would they get if they spent that on RAM

  VMware VI3 enterprise Free Hypervisor
Total Cost $17,748 $17,748
Windows Licences $5998 $5998
Virtualization licences $5750 $0
2 way Server, $6000 $6000
VMs 14 (with 2x overcommit) 63
Price per VM $1268 $281

Who’s cheapest now ? Oh look it’s Microsoft again.

Now, I mentioned the screenful of algebra and of course this is based on the same fallacious axiom that memory is the same for the two systems. This contains the same error as the first post
one line explaining the terms. MH  Physical server memory which of course is held to be the same on both systems. In practice, the Microsoft system would have more RAM and therefore better performance. I’ve reworked the equations allowing for MHV  and MHm  the memory on the VMware and Microsoft systems respectively , but keeping everything else the same.

CH Cost of server hardware
CM Cost of memory per GB
CVMW Cost of VMware virtualization software
CMS Cost of Microsoft virtualization software
COS Cost of operating system software
MHV Physical server memory, GB – Microsoft
MHM Physical server memory, GB – VM ware
MV Memory per VM, GB
r Memory overcommit ratio

The cost of the Microsoft system is CH + CM MHm + CMS +COS
And the VMware one costs CH + CM MHV + CVMW +COS

If CMS is zero and the systems cost the same then  CM MHm = CM MHV + CVMW

Solving this for memory we get  MHm = MHV + (CVMW / CM )
(In English if the cost of VM ware is $5000, and Memory costs $100 per GB, then a Microsoft system can have 50GB more memory for the same money)

The Number of Virtual Machines on VMWare is  r MHv / MV

And on Microsoft it is  MHm / MV

So to run the same number of VMs: MHm = r MHv
(in English if VMWare can run an overcommit ratio of 2 Microsoft needs twice as much memory, for the same VM count. In reality a ratio of 1.25 is more realistic, so Microsoft would need 25% more memory.)


Now we have two equations MHm = MHV + (CVMW / CM ) And : MHm = r MHv

So the break even point for VM ware comes when MHV + (CVMW / CM ) = r MHv

MHv =(CVMW / CM ) / (r -1)

if the cost of VM ware is $5000, Memory costs $100 per GB and r =2, then the VMware system needs to have 50GB of RAM and the Microsoft one 100GB of RAM: above that VMWare is cheaper, below that Microsoft is. If you think my figure of r=1.25 is closer to the real world, then the VMWare system would need 200GB of RAM (and in Eric’s scenario that means 400 VMs). Just remind me again what the memory and VM count limitations are with VMware….


Three closing points. First there are scenarios which do benefit from being able to overcommit memory (for example if you’re setting up training machines and need 100MB more memory than you’ve got)- we may have to cap the level at which it can be used to prevent customers getting themselves into trouble. But Bob Muglia has said internally and externally that the feature is needed. Whether "needed" means for tick-in-box feature comparisons or real-customer-need is open to interpretation, either way the feature will come back (it was in the Alpha versions).
Secondly Validation.Back at IT forum we pre-announced the "Server Virtualization Validation Program" personally I’m hoping that before we give customers the ability to overcommit memory or dynamically add CPUs and memory to running VMs we validate applications to give them confidence that these abilities are safe to use. If memory which a service has asked to be allocated from a non-paged pool is being paged by the virtualization stack, what will the impact be.
And finally. People tell me that VMware customers are using over commit rates of 2 or more in production the offer I made yesterday still stands. show me a customer who is running, in production, a VMware VI3 Enterprise system with a 2:1 memory overcommit ratio on all the VMs, where spending the cost of VMware on RAM wouldn’t remove the need to use overcommitment then I’ll give… lets say $270 to their choice of charity.


Technorati Tags: Windows Server 2008,Windows Server,Virtulization,Hyper-v,VMware,Fud

This post originally appeared on my technet blog.

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