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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?

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Question by:jskfan
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LVL 124
ID: 41865195
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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Author Comment

by:jskfan
ID: 41865217
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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Accepted Solution

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Andrew Hancock (VMware vExpert / EE MVE^2) earned 2000 total points
ID: 41865243
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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Author Comment

by:jskfan
ID: 41865262
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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LVL 124
ID: 41865282
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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Author Comment

by:jskfan
ID: 41865528
I believe If you reserve 25% in admission control, resources will not use 100%
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LVL 124
ID: 41865684
What was the answer to the question ?
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Author Comment

by:jskfan
ID: 41867790
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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LVL 124
ID: 41887244
do you require any further assistance to close this question?
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Author Closing Comment

by:jskfan
ID: 41888508
Thank you
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