n+1 is hogwash!

Too frequently I hear the expression n+1 as a model for ESX clusters to provide High Availability.  If you EVER expect to patch ESX servers without VM downtime then you need at least(†) n+2.  When running your clusters to only n+1, you can never safely put one of your hosts in Maintenance Mode; not if High Availability is important to you.

Footnote: If you don’t understand the importance of HA slot sizes, go learn.

3 thoughts on “n+1 is hogwash!

  1. I always try to convince the customer of n+1+1. n for the number of hosts needed to run their environment. 1 for a planned server downtime, and 1 for an unplanned server downtime.
    Only in very small or money constrained environments, does the customer choose n+1, knowing the consequences.

  2. You’re playing with semantics. What you’re really suggesting is that it’s not possible to keep HA enabled while patching an n+1 environment, which is a lot different than suggesting ESX servers cannot be patched without VM downtime in an n+1 environment.

    p.s. HA does NOT prevent VM downtime, even n+100. 🙂 (now I’m playing with semantics). You’ll need Fault Tolerance for that.

    1. Hi JC,
      Keeping HA “enabled” is somewhat mute if its useless. During patching and maintenance downtime, if you have an n+1 setup, HA may as well be off because it isn’t going to help at all. Just because it says “I’m enabled – your safe”, means diddly. I won’t work. To say that its enabled is semantics.
      The only advantage I can think of is that it saves you having to manually re-enable it once you have more redundancy in the cluster.
      But yes, I’m being a bit coarse 🙂 I want to create discussion on the subject.

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