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VMware High Availability slot calculation
Purpose
This article discusses how the VMware High Availability (HA) slot calculation has changed for the VMware vCenter Server 4.x.
Resolution
VMware HA slot calculation is done by the vCenter HA service and provides the capacity of the cluster as a whole to the various agents involved.
In VirtualCenter 2.x, the virtual machine with the maximum resource consumption was the one chosen as the basis of the slot calculation. This posed a problem if there was only one heavily resourced virtual machine and other virtual machines did not consume as many resources.
In vCenter Server 4.0, the slot size is now shown in vSphere Client on the Summary tab of the cluster.
VMware HA determines how many slots are available in each ESX/ESXi host based on the host’s CPU and memory capacity. It then determines how many ESX/ESXi hosts can fail in the cluster with at least as many slots as powered on virtual machines. If a virtual machine does not have reservations, meaning that the reservation is 0, default values of 0 MB of RAM and 256 MHz CPU speed are used.
