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VMware High Availability slot calculation (1010594)

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Purpose

This article discusses how the VMware High Availability (HA) slot calculation has changed across versions.

Resolution

VMware HA slot calculation is done by the vCenter HA service that 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 caused an issue if there was only one heavily resourced virtual machine and the other virtual machines did not consume as many resources.
 
In vCenter Server 4.0, the slot size is shown in the vSphere Client on the Summary tab of the cluster. For more information, see the VMware Availability Guide.

In vCenter Server 4.1 and 5.x, the slot size information is shown in the Summary tab of the cluster by clicking Advanced Runtime Info. For more information, see the VMware Availability Guide 4.1 and VMware vSphere Documentation for 5.x versions.
 
VMware HA determines the number of slots that are available in each ESX/ESXi host based on the host’s CPU and memory capacity. It then determines the number of  ESX/ESXi hosts that can fail in the cluster with at least as many slots as powered on virtual machines. If a virtual machine does not have reservations, for example when the reservation is 0, default values of 0 MB of RAM and 256 MHz CPU for 4.x versions and 32MHz CPU for 5.x versions. 

Note: In ESXi 5.5, the default CPU speed is 32 MHz.

Additional Information

For more information, see the Creating and Using vSphere HA Clusters section of the vSphere Availability Guide.

See Also

Update History

<pre>02/25/2013 - Added links to availability guide for ESX 40 and ESX 5.0 04/15/2014 - Update versions with VMware vCenter Server 5.1.x, VMware vCenter Server 5.5.x </pre>

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