Virtualization Oversubscription - What’s so scary? Reservations (16 of 20)

As promised on Monday here’s a quick demonstration of what a reservation does.

When both VMs want the same amount of resource (and have the same shares) they will get an even share of the CPU.  Assuming they both want all of the 4000MHz available they will each get 50% of what they want.

As the Production workload reduces Test will take more and more of the CPU, however Production will always have the rights to use 250MHz CPU.

At the point where Production is using 250MHz CPU Production is in effect getting 100% of the CPU it wants while Test is getting 93.75% of the CPU it wants.  Despite having the same shares values.

Reservations and Shares

If we run the scenario again but this time include the Shares values for the VMs the situation is different.

When they are both trying to use all of the CPU the effect of the shares will come into play and with only 1000 shares Test will get 1333MHz of the 4000MHz available while Production will get 2666MHz or Test gets 33% of what it wants to use and Production gets 66% of what it wants to use. 

As the Production workload decreases this ratio should be maintained until Production gets to its reservation.  At which point Production is in effect getting 100% of the CPU it wants while Test is getting 93.75%. I'll talk more about Reservations on Friday when we take a look at an Expandable Reservation.

In the meantime do sign up to some of our webinars, we're covering some great topics in 2017.

http://www.metron-athene.com/services/webinars/capacity-management-webinars.html

Phil Bell

Virtualization Oversubscription - What’s so scary? Reservations, Shares and Limits (15 of 20)

Reservations, Shares and Limits apply to the amount of CPU and Memory a VM or Resource pool can use.
In the example below we have an Engineering Resource Pool containing 2 Virtual Machines.

Test has 1000  CPU shares and Production has 2000 CPU shares, giving a total of 3000 shares between them.  If there is contention for CPU resource then Production will be given twice as much CPU time as Test.

Also notice the reservation on the Resource Pool has an Expandable Reservation.  This means that if there is another resource pool not using its reservation Engineering could claim and use that reservation if required.  This could cause problems if the 2^nd resource pool wishes to use its reservation as it will not be able to push Engineering out.  So while this may provide flexibility its use should be closely monitored.

On Wednesday I'll give a quick demonstration of what a reservation does.

Phil Bell

Consultant