HA dynamic admission control – bug or feature?

Admission control is part of vSphere High Availability (HA). It enforces and ensures availability in case of host failures. It guarantees that there is enough cluster capacity (memory or CPU) left for a HA failover by preventing VM power on actions that would violate that guarantee.

Since vSphere 6.5 there’s a dynamic calculation of minimum required resources, depending on your host number and host failures you want to tolerate.

Let’s start with an example: A cluster got two equal hosts and should tolerate one host failure. Admission control will make sure that neither CPU, nor memory load will exceed 50% of your total resources. If you lose one host there will be enough resources to restart VMs on the remaining host.

Let’s imagine you’re adding another two hosts to the cluster. The number of host failures to tolerate is still 1, but now dynamic resource calculation kicks in. With now four hosts, admission control will allow you to fill up the cluster to 75% before it will prevent VM power on.

That’s great. Because you just have to define your desired number of host failures to tolerate and HA admission control will dynamically calculate the allowed percentage of cluster resources to use. It works for adding and removing hosts likewise.

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ESX physical uplink resiliency

Ensure vmnic uplink redundancy with Link State Tracking / Smart Links

A vSphere cluster is redundant in many aspects. The loss of one component may not lead to a loss of functionality. Therefore we are building RAID sets from multiple disk drives, have redundant controllers in our storages, have multiple paths, redundant LAN- and SAN-switches and multiple uplinks from a host to the physical network.

VMware vSphere uses multiple physical NICs to form a logical NIC in order to gain redundancy. This is crucial for kernelports, which are responsible for vMotion, Management Network, FT, iSCSI and Heartbeats.

But there are scenarios where all vmnics have physical uplink, but a path loss further downstream towards the core lets packets wander into a black hole.

We will now discuss some network architectures and how to work around the issue.

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