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woodycollins
Contributor
Contributor

DRS Cluster Rules Question

So a question was given to me today about defining cluster rules and not being to familiar with it I thought I would ask.  What happens if you define a rule and the resources across the cluster that the rule is defined on does not have enough resources to fullfill the rule properly?  For example.  Say I have a 2 host cluster that contains 200 guest VM's.  I set a rule up that says of the 200 machines 125 machines can not run on the same server as the remaining 75.  However the resource requirements for the 125 machines exceeds the available resources for a single host.  In this example all the guest machines are already powered on and operating perfectly fine across the cluster and the rule would be implemented.

Looking for a better understanding on rules....

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a_p_
Leadership
Leadership

An interesting question. I'd say that DRS will do it's best to move the VMs to the appropriate hosts as long as there are sufficient resources on the hosts. Once the resources are exhausted no more movements will be done and you won't be able to power on additional VMs on the specific host. However this depends on the type of rule ("must" or "should" rule) you configure.

If you want to know how exactly DRS works and decides what to do, I'd recommmend you read some of the posts on http://www.yellow-bricks.com and http://frankdenneman.nl. Alternatively you may consider to purchase "vSphere 5.0 Clustering technical deepdive".

André

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a_p_
Leadership
Leadership

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woodycollins
Contributor
Contributor

Thanks for the response and thanks for moving the question into the correct thread. Smiley Happy

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hicksj
Virtuoso
Virtuoso

Depends.  Are you doing Host-VM rules or VM-VM?

Assuming you're fully automating DRS...

If VM-VM, its going to do as you request, no matter the resources.

If Host-VM, than as was stated above, you have "Must" and "Should."  "Should" will balance resources as necessary while trying to implement your requests, but will deviate from your design if an operation will cause further resource constraints.

Of course, one would have to question a 2-host cluster without the capacity to run such a configuration, though I assume you're only speaking hypothetically here...  Smiley Wink

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woodycollins
Contributor
Contributor

Yes we are in fully automating DRS, and I am talking about HOST-VM.  And the size of the cluster was just hypothetical to keep numbers simple.  The cluster we are looking at implementing such a rule is a 5 node cluster.  We should have the capacity, though we are starting to see some minor CPU contention.

Some one some were in our company has this wild idea that they would like to seperate out certain types of VM's from others (desktop VM's on seperate hosts from Server VM's).  I can see the argument both ways and haven't really put alot of thought into it.  Was just curious what would happen if we did so. Thanks for the reply's guys!

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hicksj
Virtuoso
Virtuoso

Are you a View customer?  If you're licensed for Premier where you can run as many hosts plus a vCenter server dedicated to VDI services, I would recommend splitting that off from the rest of your server environment.  View has version restriction (for vCenter, etc) that can complicate things (e.g. vSphere upgrades) in a combined environment.

If that can't be done, not sure it makes sense to spilt desktops/servers up within a cluster... other than to meet licensing obligations.   I'd think mixing the workloads would provide most bang for buck.  If there's justification to split them, I'd do so with a cluster boundry rather than DRS... seems like that would be much more managable.

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