Pre-requiste for ESXI Upgrade
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Need to enable both HA, and DRS
When vSphere HA performs failover and restarts virtual machines on different hosts, its first priority is the immediate availability of all virtual machines. vSphere HA uses the virtual machine's CPU and memory reservation and overhead memory to determine if a host has enough spare capacity to accommodate the virtual machine. After the virtual machines have been restarted, those hosts on which they were powered on might be heavily loaded, while other hosts are comparatively lightly loaded.
In a cluster using DRS and vSphere HA with admission control turned on, virtual machines might not be evacuated from hosts entering maintenance mode. You must manually migrate the virtual machines off of the hosts using vMotion. This behavior occurs because of the resources reserved for restarting virtual machines in the event of a failure.
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Disable: Allow VM power on operations that violate availability constraints
Example
VM error while migrating ( monitor progress on vCenter)
Antivirus host affinity related VMs
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Power Management need to set to OFF
vCenter Server Settings -> Advance Settings -> config.migrate.test.CompatibleNetworks.VMOnVirtualIntranet set to False
Check https://kb.vmware.com/s/article/1003832
More details
http://portal.nutanix.com/#/page/docs/details?targetId=Web_Console_Guide-NOS_v4_1:wc_cluster_hypervisor_upgrade_intro_wc_r.html