New vSphere 6.5 feature – DRS CPU Over-Commitment percentage

Over-commitment of resources is a well known feature of vSphere and allows you to use the available physical resources as efficient as possible, resulting in a possibly higher consolidation ratio (number of VM’s per ESXi host). This feature is especially interesting with regard to CPU resources, as this is a type of resource that has a very low average utilization in many server environments. Using overcommitment of CPU allows you for example to configure a number of VM’s on a host with a total of  let’s say 50 virtual CPU’s (vCPU’s) where the specific host only has 16 physical cores available. This is an example based on a general best practice to allow for a 3-to-1 overcommitment ratio (3x as many vCPU’s configured as available physical cores). Sometimes you might want to reduce this (if you have very CPU-intensive workload running on your hosts) or you could even decide to allow for a higher overcommitment ratio of 5-to-1 (for workload that uses relatively little CPU).

DRS (Distributed Resource Scheduler) is a feature of a vSphere cluster that makes sure that all workload (VM’s) running on all hosts in that cluster is provided with the resources it needs. Balancing the load within the cluster is done by using vMotion migration of VM’s from hosts that have relatively little resources to hosts where resources are more plentiful available.

Starting with vSphere 6.5 a new setting is available in DRS that allows you to configure the allowed CPU over-commitment ratio. If you enable this feature, you can configure a setting of up to 500% (a 5-to-1 over-commitment ratio).

Now … how does this work and does this have any impact on availability you may ask.

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