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We have a vSphere cluster with 4 ESXi hosts; fully automated DRS and vSphere HA is enabled on the cluster. Next to that, we have a standalone host in vCenter which is also running some VMs. Reason for having this standalone host is because some years ago we had a project that required ESXi 6.5 and our cluster was running 5.5, so we took one of the hosts out of the cluster (and vCenter) and upgrade that one to 6.5.

Last year we upgraded vCenter to 6.5 so now we should be able to add that host to our cluster again. The host is already added as standalone to vCenter. Since I'm not really a VMware guy, I'm wondering what the impact is of adding this standalone host back to the existing cluster? Will all VMs on that host keep running without interruption during the "move"? Anything else I should keep in mind?

FYI: The standalone host has access to the same datastores and networks as the hosts in the cluster. Licenses are also covered.

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The VMs will keep running, there will be absolutely no impact on them.

The VMs will automatically show up in vCenter. IIRC you can select a folder where they are placed.

For the host, it depends on where you place it in vCenter. If you create a cluster in vCenter for it, it will keep running, just with central management. If you add it to the existing cluster, it will be treated like all other hosts in the cluster, with migrations (if the CPU is compatible).

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Keep in mind that depending on the "aggressiveness" setting of DRS, you might see DRS feverishly trying to balance the cluster as soon as the host is added to the same cluster as the other hosts.

So a good strategy is to either lower the aggressiveness setting for DRS before adding the host, or to put DRS into manual mode before adding the host. This is especially important if you're dealing with a larger reshuffle among your hosts, as you usually want to get everything in it's place before DRS starts balancing things.

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