I’ve had this question twice in about a week, which means that it is time to write a quick post. How do you stop vCLS VMs from running on a vSphere HA Failover Host? For those who don’t know, a vSphere HA Failover Host is a host which is used when a failure has occurred and vSphere HA needs to restart VMs. In some cases customers (but usually Cloud partners) want to prevent the use of these hosts by any workload as it could impact the cost of usage of the platform.

Unfortunately within the UI you cannot specify that vCLS VMs cannot run on specific hosts, you can limit the vCLS VMs from running next to other VMs, but not hosts. There is however an option to specify which datastores the VMs can be stored on, and this is a potential way of limiting which hosts the VMs can run on as well. How? Well if you create a datastore that is not presented to the designated vSphere HA Failover Host then the vCLS VM cannot run on that host as the host cannot access the datastore. It is a workaround for the problem, you can find out more about the datastore placement mechanism for vCLS in this document here. Do note, as stated, those vCLS VMs won’t be able to run on those hosts, so if the rest of the cluster fails and only the Failover Host is left the vCLS VMs will not be powered on. This means that DRS will not function while those VMs are unavailable.

I’ve contacted vSphere HA/vCLS product management to see if we can get this fixed somehow more elegantly in the product, and it is being worked on.