This week I was playing with the VM-Host rules in my environment. In this particular environment I had DRS disabled and I noticed some strange things when I created the VM-Host rules. I figured it should all work in a normal way as I was always told that VM/Host rules can be configured without DRS being enabled. And from a “configuration” perspective that is correct. However there is a big caveat here, and lets look at the two options you have when creating a rule namely “should” and “must”.
When using a VM-Host “must” rule when DRS is disabled it all works as expected. When you have the rule defined then you cannot place the VM on a host which is not within the VM-Host group. You cannot power it on on those hosts, no vMotion and HA will not place the VM there either after a failure. Everything as expected.
In the case of a VM-Host “should” rule when DRS is disabled this is different! When you have a should rule defined and DRS is disabled then vCenter will allow you to power on a VM on a host which is not part of the rule. HA will restart VMs on hosts as well which are not part of the rule, and you can migrate a VM to one of those hosts. All of this without a warning that the host is not in the rule and that you are violating the rule. Even after explicitly defining an alarm I don’t see anything triggered. The alarm by the way is called “VM is violating a DRS VM-Host affinity rule”.
I reached out to the HA/DRS engineering team and asked them why that is. It appears the logic for the “should” rule, in contrary to the “must rule, is handled by DRS. This includes the alerting. It makes sense to a certain extent, but it wasn’t what I expected. So be warned, if you don’t have DRS enabled, “VM-Host should rules” will not work. Must rules however will work perfectly fine. (Yes, I’ve asked them to look in to this and fix it so it behaves as you would expect it to behave and come with a warning when you try anything that violates a should rule.)
Thanks Duncan. Your posts are always right on time and (usually) offer the exact insights I am looking for.
I even would have expected that disabling DRS would also disable any rules. Why is a host-VM “must” rule still working with DRS disabled? What is the thought behind it?
Duncan Epping says
Because there is also HA which can place VMs… and HA adheres the Must Rules 🙂
It is not just DRS , there is HA also which places VMs .