Monday 9 January 2017

Virtualization Oversubscription - What’s so scary? Why are we interested in queuing? (20 of 20)

The reason we were talking about queueing theory is that it’s part of how the hypervisor copes with CPU oversubscription, by queueing the VMs.

VMs Queue for free CPUs

·        Ready Time

·        Co-Stop time

·        Higher utilization = higher contention

·        More concerned about CPU busy than vCPU to logical CPU ratio

You can see when this starts to happen by monitoring ready and Co-Stop metrics.  You should typically be more worried about CPU busy than you are the ratio of CPUs in the VMs to the logical CPUs presented by the hardware.

Because all this is Math, people have written programs to model this, so you can see how busy you can run your hosts before performance becomes unacceptable.
Summary

If there is anybody reading this who considered oversubscription to mean poor performance then hopefully I’ve gone some way to showing you that’s not the case.

        Oversubscription does not equal unacceptable performance

        Virtualisation is expecting you to oversubscribe

       It’s the reason it exists (Don’t throw that away.  It costs you money!)

        Take the fear out of oversubscription through proper planning

       Plan for performance, not ratios

Look at the metrics on your systems and use them to model the point where performance will degrade because of utilization.  You cannot do that by looking at the ratio of vCPUs to logical CPUs but you can with utilization figures. Thanks for following my blog series, if you have any questions please feel free to drop me a line.
If you'd like to learn more about managing VMware Capacity then why not try our Pay-to-View workshop
Phil Bell
Consultant

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