VMware DRS Rules

VMware DRS Rules

1.    Must run rules (Mandatory)

A mandatory rule limits HA, DRS and the user in such a way that a virtual machine may not be powered on or moved to a ESX host that does not belong to the associated DRS host group.

2.    Should run rules (Preferential)

A preferential rule defines a preference to DRS to run virtual machine on the host specified in the associated DRS host group.

·        Must run on hosts in group:

·        The VM Group must run on the hosts in this group. If the selected hosts are down, the VMs will be down and not be restarted on a different host.

·        If you have applications with special license agreements, you might have to use this option.

·        Should run on hosts in group:

·        The VM Group should run on the hosts in the group. However, in case of a vSphere HA event, this rule will be overwritten in order to keep the VMs running.

·        Must Not run on hosts in group:

·        The VM Group will not run on the specific hosts group. Under no circumstances will the VMs be moved to the specified host group. The VMs will rather be down than moved to this host group.

·        Should Not run on hosts in group:

·        The VM Group should not run on the hosts in the group. However, in case of a vSphere HA event, this rule will be overwritten in order to keep the VMs running.

The way HA treat preferential rules?
VMHA respects obey mandatory rules when placing virtual machines after a host failover.

·       It can only place virtual machines on the ESX hosts that are specified in the DRS host group.

·       DRS does not communicate the existence of preferential rules to HA, therefore HA is not aware of these rules. HA cannot prevent placing the virtual machine on a ESX host that is not a part of the DRS host group, thereby violating the affinity rule. DRS will correct this violation during the next invocation.

DRS preferential rules?
During a DRS invocation, DRS runs the algorithm with preferential rules as mandatory rules and will evaluate the result. If the result contains violations of cluster constraints; such as over-reserving a host or over-utilizing a host leading to 100% CPU or Memory utilization, the preferential rules will be dropped and the algorithm is run again.

Finally it depends on our requirement for Service and Infrastructure availability but the most important thing is to have a clear understanding for this rules before implementing them.

Hope this was informative…..

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