High Availability (HA) slot size calculation is an essential part of VMware vSphere’s HA feature. HA slot size determines the number of virtual machines that can be powered on per ESXi host in a VMware HA cluster without violating the resource reservations and constraints. Proper slot size calculation ensures that there is sufficient capacity to restart virtual machines on other hosts in the event of a host failure.

To calculate the HA slot size, follow these steps:

Step 1: Gather VM Resource Requirements:

• Identify all the virtual machines in the VMware HA cluster.
• For each VM, determine its CPU and memory reservation or limit. If there are no reservations or limits, consider the VM’s configured CPU and memory settings.

Step 2: Identify the Host with the Highest CPU and Memory Resources:

• Determine the ESXi host in the cluster with the highest CPU and memory resources available (CPU and memory capacity).

Step 3: Calculate the HA Slot Size: The HA slot size is calculated using the following formula:

Slot Size = MAX ( CPU Reservation, CPU Limit, CPU Configuration ) + MAX ( Memory Reservation, Memory Limit, Memory Configuration )

• MAX (CPU Reservation, CPU Limit, CPU Configuration): Identify the highest value among the VMs’ CPU reservations, CPU limits, and CPU configurations.
• MAX (Memory Reservation, Memory Limit, Memory Configuration): Identify the highest value among the VMs’ memory reservations, memory limits, and memory configurations.

Step 4: Determine the Number of HA Slots per ESXi Host:

• Divide the total available CPU resources and memory resources of the identified ESXi host by the calculated HA slot size.
• Round down the result to get the number of HA slots per ESXi host.

Step 5: Calculate the Total Number of HA Slots for the Cluster:

• Multiply the number of HA slots per ESXi host by the total number of ESXi hosts in the VMware HA cluster to get the total number of HA slots for the cluster.

Step 6: Determine the Maximum Number of VMs per Host:

• Divide the total number of HA slots for the cluster by the total number of ESXi hosts in the cluster to get the maximum number of VMs that can be powered on per host.

Example: Suppose you have a VMware HA cluster with three ESXi hosts and the following VM resource requirements:

VM1: CPU Reservation = 2 GHz, Memory Reservation = 4 GB VM2: CPU Limit = 3 GHz, Memory Limit = 8 GB VM3: CPU Configuration = 1 GHz, Memory Configuration = 6 GB

ESXi Host with the Highest Resources: CPU Capacity = 12 GHz, Memory Capacity = 32 GB

Step 3: Calculate the HA Slot Size:

• CPU: MAX(2 GHz, 3 GHz, 1 GHz) = 3 GHz
• Memory: MAX(4 GB, 8 GB, 6 GB) = 8 GB

Slot Size = 3 GHz + 8 GB = 11 GHz, 8 GB

Step 4: Determine the Number of HA Slots per ESXi Host:

• CPU: 12 GHz (ESXi host CPU capacity) / 11 GHz (Slot Size) ≈ 1.09 (Round down to 1)
• Memory: 32 GB (ESXi host memory capacity) / 8 GB (Slot Size) = 4

Step 5: Calculate the Total Number of HA Slots for the Cluster:

• Total HA Slots = 1 (HA slots per ESXi host) * 3 (number of ESXi hosts) = 3

Step 6: Determine the Maximum Number of VMs per Host:

• Maximum VMs per Host = 3 (Total HA Slots) / 3 (number of ESXi hosts) = 1

In this example, each ESXi host can run up to one VM at a time without violating resource constraints.

Keep in mind that the HA slot size calculation is a conservative estimate to ensure enough resources are available for VM restarts. As a result, some resources might be underutilized, especially if there are VMs with large reservations or limits. It is essential to review and adjust VM resource settings as needed to optimize resource utilization in the VMware HA cluster.