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Vmware Affinity Rules

Posted on 2016-10-29
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When configuring Affinity rules so that 2 VMs will always stay on the same ESX host, then the host fails and the VMs will not find another ESX host that has enough resources for the 2 VMs together, what will happen ? and how do you ensure that there is always enough resources on other ESX hosts for the 2 VMs together?

Thank you
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Question by:jskfan
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by:Andrew Hancock (VMware vExpert / EE MVE)
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When configuring Affinity rules so that 2 VMs will always stay on the same ESX host, then the host fails and the VMs will not find another ESX host that has enough resources for the 2 VMs together, what will happen ?

depends on how rigid you have Admission Control configured, if you have it configured such that it breaks the rule, the VMs will not be allowed to startup on other hosts, because it would break the rules.

and how do you ensure that there is always enough resources on other ESX hosts for the 2 VMs together?

Management and Checking Capacity.

see here

https://pubs.vmware.com/vsphere-55/index.jsp#com.vmware.vsphere.resmgmt.doc/GUID-793013E2-0976-43B7-9A00-340FA76859D0.html
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by:jskfan
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If you reserve CPU/MEMORY  capacity: 25%
then will the Cluster be able to move around other VMs between ESX hosts to make room for the 2VMs together on one ESX host ?
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Andrew Hancock (VMware vExpert / EE MVE) earned 500 total points
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Again it depends on your Admission Rules, and how they are set.

If you have Admission Rules set as default, it will not violate them, and VMs will not failover, so you will have to ensure, if you want VMs to startup on other hosts, to have them disabled.

You will need to ensure you have the resources available to fail over.

If you reserve CPU/MEMORY  capacity: 25%

How ?

Depends on your current loading, how many hosts do you currently have in your Cluster ?
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by:jskfan
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To simplify the example:
Let's say I have 4 Hosts with 4GHZ and 4GB RAM each

I have 8 Vms  on the Cluster. each VM is allocated 2GB RAM and 2GHZ CPU
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by:Andrew Hancock (VMware vExpert / EE MVE)
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Okay, so you have 4 Hours, with 4GHz and 4GB RAM, RAM will always be the bottleneck.

You have 8 VMs, I assume, these are allocated as

Host 1 - 2 VMs with 2GB and 2GHz - Total 4GB and 4GHz
Host 2 - 2 VMs with 2GB and 2GHz -Total 4GB and 4GHz
Host 3 - 2 VMs with 2GB and 2GHz -Total 4GB and 4GHz
Host 4 - 2 VMs with 2GB and 2GHz - Total 4GB and 4GHz

If also the assumption is that ALL Host are 100% Utilised

So if a Host A was to fail, what do you think would happen ?

In a standard created HA Cluster with Default Settings ?
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by:jskfan
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I believe If you reserve 25% in admission control, resources will not use 100%
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by:Andrew Hancock (VMware vExpert / EE MVE)
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What was the answer to the question ?
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by:jskfan
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I meant with 25% reserved in Admission Control then,  that will guarantee the 2 VMs can stay on one host if they get vmotioned
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by:Andrew Hancock (VMware vExpert / EE MVE)
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do you require any further assistance to close this question?
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by:jskfan
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Thank you
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