VMware DRS Rules – Should VS Must

Most of us are using VMware DRS and I think it is actually one of the coolest features. Why manually vMotion VMs between hosts, when you can use the Distributed Resource Scheduler and just let it migrate VMs automatically.

Some of you might think, what if I want to have particular VMs running group of hosts? Well, this is where VMware DRS Rules come into play. DRS Rules allow you to group your VMs and hosts, which are within the same vSphere HA cluster and specify which should or must run on what hosts.

When setting up DRS Rules, you have the following choices:

  • 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.

If you are following my lead, go with “Should run on hosts in group” or “Should Not run on hosts in group” except if you cannot avoid it. If you do not have any special license agreements, which forbid you to migrate a certain application to different hosts, I do not see a good reason to use either of the other two rules available.

I would love to hear from you, why you are using “Must run on hosts in group” or “Must Not run on hosts in group” rather than the other two rules.

10 Comments|Add your own comment below

    1. For resource pools you have four different options:

      Low: 500 shares per virtual CPU and 5 shares per megabyte of configured virtual machine memory

      Normal: 1000 shares per virtual CPU and 10 shares per megabyte of configured virtual machine memory

      High: 2000 shares per virtual CPU and 20 shares per megabyte of configured virtual machine memory

      If you select “Custom” you can specify your own amount of shares.

  1. Hi Jan,
    Thank you for above post,
    I got a clarity on explanation of mandatory and preferential Rules on the DRS cluster.
    However can you give me some practice examples, so that I can be much confident where this can be applicable under which circumstances.

    Regards
    Raj Navalgund
    Sr VMware and windows AD administrator.
    Bangalore
    Karnataka–India.

  2. Thanks, Jan. I set up a Should Rule because I want the VMs in question to autostart in an HA event, but 1 VM didn’t comply and move over. I was wondering if host resources would be a factor (this host is slightly smaller and definitely memory constrained).

    But! It turns out someone had a conflicting DRS rule.

    Thanks for the info!

  3. Hi Jan,

    If you can update here, what are new features of DRS and HA of latest vsphere version are supporting. wouldn’t that interesting 🙂

    Raj Navalgund

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