Example Architectural Decision – HA Admission Control Policy with Software licensing constaints

High Availability Admission Control Setting & Policy with a Software Licensing Constraint

Problem Statement

The customer has a requirement to virtualize “Application X” which is currently running on physical servers. The customer is licensed for a maximum of 32 cores and the software vendor has strict licensing restrictions which do not recognize the use of DRS rules to restrict virtual machines to a sub-set of hosts within a cluster.

The application is Tier 1, and requires maximum availability. A capacity planner assessment has been conducted and found 32 cores and 256Gb RAM is sufficient to run all servers.

The servers requirements vary greatly from 1vCPU/2GB RAM to 8vCPU/64GB Ram with the bulk of the VMs 2vCPU or less with varying RAM sizes.

What is the most suitable hardware configuration and HA admission control policy / setting  that complies with the licensing restrictions while ensuring N+1 redundancy and minimizing the change of poor application performance?


1. None


1. Software vendor has strict licensing requirements
2. Only 32 cores are licensed and the customer has no budget for further licenses
3. DRS rules cannot be used to isolate VMs onto one or more hosts due to software licensing agreement


1. Ensure maximum availability for the Tier 1 application/s
2. Ensure optimal performance for Tier 1 application/s

Architectural Decision

Purchase a total of three (3) x Two (2) Way Servers, with 8 core CPUs and 128GB Ram each and form a cluster of three nodes.

For the HA Admission control setting use “Enable – Do not power on virtual machines that violate availability constraints”

For the HA admission control policy use “Specify a Failover Host” and select the third host in the cluster. (Leaving two active hosts in the cluster).


1. Enabling strict admission control is critical to ensure the required level of availability for the Tier 1 application
2. Ensure maximum CPU scheduling efficiency by having two hosts active within the cluster running virtual machines as opposed to a single large host
3. Having 2 active hosts in the cluster allows DRS some flexibility to load balance to resolve contention compared to using a single large 32 core host
4. N+1 redundancy is achieved as one host can fail and the “fail-over” host will become active and be able to take the failed hosts workloads without performance degrading
5. As only 32 cores ( 2 servers with 16 cores each) are active at any one time, the solution complies with the licensing constraint
6. Using CPUs with smaller numbers of cores (such as 5 x 2 way servers with 4 cores per socket) would result in larger VMs not fitting within NUMA nodes and potentially impacting memory performance. Although, with vNUMA in vSphere 5.0 this would be less of an issue.
7. All VMs will fit within a NUMA node thus giving the VMs maximum performance without the requirement for vNUMA which is only available in vSphere 5.0 or later
8. The compute resource supplied by the proposed cluster is sufficient to run the workloads as per the capacity planner assessment.


1. Additional networking and storage ports for three hosts as opposed to a two host cluster
2. If additional compute is required in the cluster, additional software licenses would need to be purchased. Alternativley if the application servers were redesigned to use a scale out methodology (especially for VMs with 4-8vCPUs) it would likley result in higher overcommitment ratios without significant contention and better utilization of the existing licensed cores
3. One host is sitting as a hot standby not servicing customer workloads and may be considered to be “waste”


1. Use 2 x 4 way 8 core ESXi hosts (32 cores per host) and set HA admission control to specify a fail over host
2. Use 5 x 2 Way 4 core ESXi hosts (8 cores per host) and set HA admission control to specify a fail over host

The Below is a basic diagram of the proposed solution.


*Post updated February 11th to correct an error.

3 thoughts on “Example Architectural Decision – HA Admission Control Policy with Software licensing constaints

  1. The “Specify a Failover Host” Admission Control option doesn’t use slots or calculate a slot size, so there is no need or requirement to limit the size of the slot using the advanced options. The only Admission Control option that these settings would affect is the “Host failures the Cluster Tolerates” option.