If a Primary VM appears to be executing slowly, even though its host is lightly loaded and retains idle CPU time, check the host where the Secondary VM is running to see if it is heavily loaded.

Problem

When a Secondary VM resides on a host that is heavily loaded, this can effect the performance of the Primary VM.

Evidence of this problem could be if the vLockstep Interval on the Primary VM's Fault Tolerance panel is yellow or red. This means that the Secondary VM is running several seconds behind the Primary VM. In such cases, Fault Tolerance slows down the Primary VM. If the vLockstep Interval remains yellow or red for an extended period of time, this is a strong indication that the Secondary VM is not getting enough CPU resources to keep up with the Primary VM.

A Secondary VM running on a host that is overcommitted for CPU resources might not get the same amount of CPU resources as the Primary VM. When this occurs, the Primary VM must slow down to allow the Secondary VM to keep up, effectively reducing its execution speed to the slower speed of the Secondary VM.

Results

To resolve this problem, set an explicit CPU reservation for the Primary VM at a MHz value sufficient to run its workload at the desired performance level. This reservation is applied to both the Primary and Secondary VMs ensuring that both are able to execute at a specified rate. For guidance setting this reservation, view the performance graphs of the virtual machine (prior to Fault Tolerance being enabled) to see how much CPU resources it used under normal conditions.