VM-Host Affinitiy Rule

As we all know that we have two type of VM-Host Affinity Rule. This rules tell whether or not a group of VM’s can run on Specified DRS group

1. Mandatory Rules (Must)2. Preferred Rules (Should)

Mandatory Rules: It is must rule for HA, DRS and DPM, so when a VM is placed in ESX host specified in the DRS group it prevent VM from running on them.Preferred Rules : It is a preferred rule for DRS and DPM and it will try his best by not running the VM in ESX host specified in the DRS group but HA  would restart the VM on the other host in DRS group in case of a host failure.Below table summarize which rule is respected by HA, DRS and DPM

Rule Type

Affinity Anti-Affinity  DRS DPM

HA

VM-Host

Should run on hosts in group

Should not run on hosts in group

No No No

Must run on hosts in group

Must not run on hosts in group

Yes Yes Yes

So now the question is how HA Know which VM’s are specified in Must rule??Well we know HA maintains a ‘compatlist‘ file  which contains the protected VM’s list and their compatible hosts. The file is located under /etc/opt/vmware/fdm/