This section describes the DRS cluster sizing and resource settings.
Carefully select the resource settings (that is, reservations, shares, and limits) for your virtual machines.
Setting reservations too high can leave few unreserved resources in the cluster, thus limiting the options DRS has to recommend vSphere vMotion migrations.
Setting limits too low could keep virtual machines from using extra resources available in the cluster to improve their performance.
Use reservations to guarantee the minimum requirement a virtual machine needs, rather than what you might like it to get. Note that shares take effect only when there is resource contention. Note also that additional resources reserved for virtual machine memory overhead need to be accounted for when sizing resources in the cluster.
If the overall cluster capacity might not meet the needs of all virtual machines during peak hours, you can assign relatively higher shares to virtual machines or resource pools hosting mission-critical applications to reduce the performance interference from less-critical virtual machines.
If you will be using vSphere vMotion, it’s a good practice to leave some unused (and unreserved) CPU capacity in your cluster. As described in vSphere vMotion Recommendations, when a vSphere vMotion operation is started, VMware Cloud on AWS reserves some CPU resources for that operation.
