VMware DRS Rules


VMware DRS Rules
1 .       Must operate rules (Mandatory)
A mandatory guideline limits HA, DRS and the consumer in such a way that a virtual machine might not be powered on or moved to the ESX host that does not belong to the particular associated DRS host group.
2 .       Need to run rules (Preferential)
The preferential rule defines a choice to DRS to run virtual device on the host specified in the related DRS host group.
·               Must   run on hosts in group:
·               The VM Group  must  operate on the hosts in this group. When the selected hosts are down, the particular VMs will be down and not end up being restarted on a different host.
·               If you have applications with special permit agreements, you  might  have to make use of this option.
·               Should   run on hosting companies in group:
·               The VM Group  should  run on the hosting companies in the group. However , in case of the vSphere HA event, this principle 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 particular VMs be moved to the specific host group. The VMs may rather be down than relocated to this host group.
·               Should Not  run on hosts within group:
·               The VM Group  should not  run on the serves in the group. However , in case of the vSphere HA event, this principle will be overwritten in order to keep the VMs running.



The way ST?LLA TILL MED ETT treat preferential rules?
VMHA values obey mandatory rules when putting virtual machines after a host failover.
·             It can only place virtual devices on the ESX hosts that are specific in the DRS host group.
·             DRS does not communicate the existence of preferential guidelines to HA, therefore HA is not really aware of these rules. HA are unable to prevent placing the virtual device on an ESX host that is not part of the DRS host group, therefore violating the affinity rule. DRS will correct this violation throughout the next invocation.
DRS preferential rules?
During a DRS invocation, DRS runs the algorithm with preferential rules as mandatory rules and can evaluate the result. If the result includes violations of cluster constraints; for example over-reserving a host or over-utilizing a number leading to 100% CPU or Storage utilization, the preferential rules is going to be dropped and the algorithm is operate again.
Finally it depends on our requirement of Service and Infrastructure availability however the most important thing is to have a crystal clear understanding for this rules before applying them.
Hope this was informative…..
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