vSphere High Availability (HA) ensures availability of the virtual machines in your SDDC. If a host fails, vSphere HA restarts its VMs on a different host. All clusters in a VMware Cloud on AWS SDDC are configured to use vSphere HA. This setting cannot be reconfigured.

To ensure availability of all workload and management VMs in your SDDC, VMware Cloud on AWS must maintain sufficient capacity to power them on in the event of host failure. HA admission control is the primary mechanism for capacity maintenance. Admission control imposes constraints on resource usage, and can prevent any action that consumes more resources than the cluster can support during a failover. These constraints apply to actions like powering on or migrating a VM, or reserving additional CPU or memory resources for a VM, and effectively limit the availability of host resources as shown here:
  • In a two-host i3.metal SDDC cluster, admission control prevents you from powering-on more than 36 VMs or assigning more than 1152 MHz CPU reservation to a single VM.
  • In SDDC clusters with three to five hosts, admission control reserves one host for failover.
For additional details about system limits, see VMware Configuration Maximums.

For a detailed discussion of vSphere HA and other features that ensure vSphere availability, see How vSphere HA Works in the VMware vSphere Product Documentation. Although most HA settings in VMware Cloud on AWS are managed for you by VMware and cannot be changed in your SDDC's vCenter, it's important to understand the fundamental concepts of vSphere HA and how they apply to workload deployment in your SDDC.


  1. In the vSphere Client, click Menu and select Hosts and Clusters.
  2. Select Cluster-1.
    In VMware Cloud on AWS, Cluster-1 holds configuration settings for all SDDC clusters.
  3. (Optional) Examine the vSphere HA settings, which are optimized for VMware Cloud on AWS.
    Although you cannot modify these settings, familiarity with them makes it easier for you to understand how VMs are deployed across the clusters in your SDDC.
  4. Click the vSphere HA Monitoring link for more information on vSphere HA events.