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.
Phil Bell
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