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Admission control settings on metro availability

  • 11 April 2017
  • 3 replies
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Hi guys,
My configuration is one stretch vsphere 6 cluster across two sites, evey site with 1xNX-1065S with 3 nodes.
Due to offices renovations on primary site, last Friday i started the failover procedure to the secondary site.
So far so good until the VM scheduled restart during w.end, some VM's can't start due to admission control resources violation.
I've check the original settings and is 33%, i disable it and manually start the missing VM's.
I check the metro availability guide updated on January 2017:

Configure VMware HA admission control based on the cluster configuration.
⁃ Assuming a balanced configuration, one option is to set “Percentage of cluster resources
reserved as failover spare capacity” to 50 percent or lower. This setting ensures that
enough resources exist in a single Nutanix cluster to support both sites.

Can you confirm the correct valus is 50%?

Thanks in advance
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Best answer by mmcghee 12 April 2017, 12:53

Hi Cipo80



Admission control can be useful, but as you point out it's really dependent on your environment. The 50% example in the guide is assuming loss of 50% of the nodes within the cluster in a worst case scenario (site failure) in a symmetrical configuration.



In other cases, an environment may not be replicating all VMs between both sites, so even though you may lose half the nodes, you may not need 50% of total resources to run the VMs you've replicated.



Further, you can have an asymmetric configuration, where total compute resources may differ between the two sites.



Ultimately it's about reserving enough resources to run the virtual machines you intend to replicate and run should one of your sites completely fail.



Thanks,

Mike
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3 replies

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Hi, i think 50% isn't correct, because during failover i can lose 1 esx node with no data distruption and sufficient CPU/RAM resource to work with all VM's.
66% in my scenario maybe correct, but the question is admission cotrol is helpful or useless?
Userlevel 4
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Hi Cipo80

Admission control can be useful, but as you point out it's really dependent on your environment. The 50% example in the guide is assuming loss of 50% of the nodes within the cluster in a worst case scenario (site failure) in a symmetrical configuration.

In other cases, an environment may not be replicating all VMs between both sites, so even though you may lose half the nodes, you may not need 50% of total resources to run the VMs you've replicated.

Further, you can have an asymmetric configuration, where total compute resources may differ between the two sites.

Ultimately it's about reserving enough resources to run the virtual machines you intend to replicate and run should one of your sites completely fail.

Thanks,
Mike
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Thank you Mike for the reply