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Non-Uniform configurations for VSAN clusters

Duncan Epping · Nov 6, 2014 ·

I have been receiving various questions around support for non-uniform configurations in VSAN environments (sometimes also referred to as “unbalanced” configurations) . I was a bit surprised by it to be honest as personally I am not a big fan of non-uniform configurations to begin with. First, with “non-uniform” I am referring to different hardware configurations. In other words you have four hosts with 400GB Intel s3700 flash and one host with 200GB Intel s3500 flash. The question was if this is an acceptable configuration if the overall flash capacity still meets the recommended practice of 10% of used capacity.

Although technically speaking this configuration will work and is supported, from an operational and user experience perspective you need to ask yourself if this is a desired scenario. I have seen people doing these type of constructions out in the field as well with “flash caching” solutions and believe me when I say that the result were very mixed. The problem is that when you have a non-uniform configuration your predictability of performance will be impacted. As you can imagine cutting your flash capacity in half on a host could impact the cache hit ratio for that particular host. Also using a different type of flash will change your results / experience more then likely. On top of that, imagine you need to do maintenance on your hosts, it could be that the “non-uniform” host will have different procedures for whatever maintenance you are doing… it just complicates things unnecessarily.

So again, although this is supported and will work from a technical perspective it is not something I would recommend from an operational and user experience point of view.

Related

Server, vSAN virtual san, VMware, vsan, vSphere

Reader Interactions

Comments

  1. Josemhernandez says

    12 November, 2014 at 00:33

    Good appreciation to consider in future server upgrades.

    Thanks for your help.

  2. Anthony says

    10 July, 2015 at 21:47

    We are using IBM x3650 M4 hosts with 16 drive slots. After subtracting 2 drives for boot, this leaves 14 slots. Our question is, could we create “uneven” disk group of 5-5-4? Yes, we could do 7-7 but large SSDs are still quite expensive though. Thanks

    • Duncan Epping says

      11 July, 2015 at 17:49

      I don’t think 5-5-4 would be too big of a deal and I would recommend doing that.

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About the author

Duncan Epping is a Chief Technologist in the Office of CTO of the Cloud Platform BU at VMware. He is a VCDX (# 007), the author of the "vSAN Deep Dive", the “vSphere Clustering Technical Deep Dive” series, and the host of the "Unexplored Territory" podcast.

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