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Adjust VMware High Availability Slot Size in the VMware vSphere Web Client

I’ve been using the VMware vSphere web client since it was released, and I’ve found that it offers plenty of features that are useful for VMware admins. One hidden gem in the vSphere Web Client is the feature that allows you to adjust the High Availability (HA) slot size. This was not something possible in the fat client versions and was automatically done by vCenter based on VM size. In this post I will explain how to adjust the HA slot size setting in the vSphere web client.

What Is High Availability (HA) Slot Size?

The definition of a slot is the logical representation of CPU and memory resources required to satisfy the requirements of all powered-on virtual machines in a cluster.

This is basically the worst case CPU and memory scenario for any given VM in the selected cluster. The defined slot is used when Admission Control is configured for the cluster and “Host Failures Tolerates” has been configured as the admission control policy. To figure out the number of VMs that can be powered on without exceeding resources, the cluster divides the available resources by the slot size. This provides the number of VMs that could be powered on.

How to Adjust HA Slot Size

When turning on vSphere HA and configuring Admission Control in the vSphere web client you will notice a new option for changing the Slot size policy. This was not possible in earlier version of vSphere. In the example shown below, I turned on HA and chose the option to configure a fixed slot size. This allows me to adjust per my requirements rather than letting the system auto configure.

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This shows that I have 31 VMs on this cluster and that they are using a small amount of CPU, but that the memory requirements are high due to a single VM. There is also an option that shows a count of any VMs that would require multiple slots.


Adjust High availability HA Slot Size


In this sample I have elected to adjust the memory setting for the slot size. I have adjusted it down from 6GB to 2GB. On this cluster I have 30 VMs configured with 2GB of memory and one VM with 6GB of memory. The one large VM was what drove the large slot size in the previous example. After adjusting the memory size down for the slot, you now see that the count for VMs requiring multiple slot sizes now shows 1/31. That represents the one large VM that will need more than one slot to start up.

Adjust High availability HA Slot Size


Click on the View option from the VMs requiring multiple slots from the previous image and a window opens (shown below) showing a list of any virtual machines that require multiple HA slots.

vms requiring multiple shots


Adjusting HA slot size on your clusters is a delicate configuration that should not be taken lightly. Before making any adjustments to slot size or even choosing the right HA method, I suggest that you explore and understand all options. Then look at your environment and make the choice that fits your configuration.

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