Vmware Slot Size Policy

Vmware Slot Size Policy Chart

vSphere HA uses admission control to ensure that sufficient resources are reserved for virtual machine recovery when a host fails.

Admission control imposes constraints on resource usage. Any action that might violate these constraints is not permitted. Actions that might be disallowed include the following examples:

The first configuration parameter you can set is the size of an individual virtual machine's memory. Set this configuration parameter for the virtual machine in the virtual machine settings editor (VM Settings Memory). The guest memory size should not be set lower than the minimum recommendations of the operating system provider. The Host Failures Cluster Tolerates admission control policy is based on the calculation on a slot size comprised of two components, the CPU and memory reservations of a virtual machine.

• Powering on a virtual machine
• Migrating a virtual machine
• Increasing the CPU or memory reservation of a virtual machine

• Cluster resource percentage
• Slot policy
• Dedicated failover hosts

Regardless of the admission control option chosen, a VM resource reduction threshold also exists. You use this setting to specify the percentage of resource degradation to tolerate, but it is not available unless vSphere DRS is enabled.

The resource reduction calculation is checked for both CPU and memory. It considers a virtual machine's reserved memory and memory overload to decide whether to permit it to power on, migrate, or have reservation changes. The actual memory consumed by the virtual machine is not considered in the calculation because the memory reservation does not always correlate with the actual memory usage of the virtual machine. If the actual usage is more than reserved memory, insufficient failover capacity is available, resulting in performance degradation on failover.

Setting a performance reduction threshold enables you to specify the occurrence of a configuration issue. For example:

• The default value is 100%, which produces no warnings.
• If you reduce the threshold to 0%, a warning is generated when cluster usage exceeds the available capacity.
• If you reduce the threshold to 20%, the performance reduction that can be tolerated is calculated as performance reduction = current utilization * 20%. When the current usage minus the performance reduction exceeds the available capacity, a configuration notice is issued.

After you create a cluster, you can configure admission control to specify whether virtual machines can be started if they violate availability constraints. The cluster reserves resources so that failover can occur for all running virtual machines on the specified number of hosts.

Vmware Slot Size Policy Guide

The Admission Control page appears only if you enabled vSphere HA.

Vmware Slot Size Policy Comparison

Procedure

1. In the vSphere Client, browse to the vSphere HA cluster.
2. Click the Configure tab.
3. Select vSphere Availability and click Edit.
4. Click Admission Control to display the configuration options.
5. Select a number for the Host failures cluster tolerates. This is the maximum number of host failures that the cluster can recover from or guarantees failover for.
6. Select an option for Define host failover capacity by. Option

   Description
   Cluster resource percentage	Specify a percentage of the cluster’s CPU and memory resources to reserve as spare capacity to support failovers.
   Slot Policy (powered-on VMs)	Select a slot size policy that covers all powered on VMs or is a fixed size. You can also calculate how many VMs require multiple slots.
   Dedicated failover hosts	Select hosts to use for failover actions. Failovers can still occur on other hosts in the cluster if a default failover host does not have enough resources.
   Disabled	Select this option to disable admission control and allow virtual machine power ons that violate availability constraints.

7. Set the percentage for the Performance degradation VMs tolerate.
   This setting determines what percentage of performance degradation the VMs in the cluster are allowed to tolerate during a failure.
8. Click OK.

Vmware Slot Size Policy Tool

Your admission control settings take effect.