Some vCenter Basics and an error when trying to add a host to cluster

This post is not for anyone who is reasonably experienced with ESXi and vCenter. Nope. It’s for me, the noob.

I am marginally more sophisticated than an army of monkeys on keyboards trying to manage a vSphere environment – just trying to set up a server I have under my desk so that I have an environment in which I can play with all things BOSH. I don’t want to become an expert or anything, just want a simple system that provides basic functionality.

I got ESXi upgraded to 6.0, installed a fresh vCenter Server and then was trying to set up some really basic constructs – datacenters, clusters and the like. And I’ve be stumbling from one obstacle to the next so let me capture a few tips here that I wish I’d been able to find while googling:
  • The vCenter server appliance can exist completely independently from any of your ESXi hosts. This took me just a little bit to grok as I first set up my ESXi host and then moved onto installing vCenter “on it.” But you shouldn’t think of it that way – vCenter is just the management interface for your vSphere environment and when I did its installation, the fact that I was doing anything with the ESXi host is just a matter of convenience – it just so happens that my ESXi host will allow me to have a VM, and it just so happens that what I want to install on said VM is the vCenter Server Appliance. Installing vCenter “onto” my ESXi host is not extending that host, or customizing it, or configuring it – what we have are really two logically independent things – the ESXI host, and an application for managing an environment that I just so happened to run on a VM that is hosted on my ESXi server.
  • vCenter allows a user to work with a set of abstractions for your data center – in fact, “datacenter” is one of those abstractions. A datacenter holds a bunch of other things like clusters, data stores, networks, etc. That datacenter is purely logical – when you create one you are really just creating a “bucket” that will hold all of those other things. When you create this datacenter you are not changing a single thing about your ESXi host. Setting up and managing your data center, and all of the things it contains, is the central thing you will be doing with vCenter.
  • One of types of things that a datacenter will contain is an ESXi host . (“Ah ha” moment)
  • A host can exist in the datacenter directly, or it can be part of a cluster.
  • Right after you create a datacenter you might be gently guided to add a host to that data center. I was, and so I added my ESXI host to that data center.
  • For one of the things I want to install into my ESXi environment I need to have a cluster – so I created it. A cluster lives in a datacenter and it is a collection of hosts.
  • I then went on to add my host to that cluster and it failed with the error “The name already exists”.
    • Here’s why – a host can only exist in one place in the datacenter – it either exists directly in the datacenter, or it exists within a cluster. Sure, in retrospect this is quite obvious, but prior to having the basic understanding of what vCenter is about, and the key abstractions it provides, it was far less so. And the error message is far from clear – I kept looking at my empty list of hosts in the cluster thinking “Uh, no. There is no other host in the list with that name.” Point is that the namespace for these host names is across all clusters in the datacenter, as well as hosts that are added directly to the datacenter.
  • So the solution was to remove the host from the datacenter after which I was able to add it to the cluster.
Hope this helps another newbie at some point.

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