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Unbalanced Stretched vSAN Cluster

Duncan Epping · Jul 30, 2017 ·

I had this question a couple of times the past month so I figured I would write a quick post. The question that was asked is: Can I have an unbalanced stretched cluster? In other words: Can I have 6 hosts in Site A and 4 hosts in site B when using vSAN Stretched Cluster functionality? Some may refer to this as asymmetric or uneven. Either way, the number of hosts in the two locations differ.

Is this supported? In short: Yes.

The longer answer: Yes you can do this, this is fully supported but you will need to keep your selected PFTT (Primary Failures To Tolerate), SFTT (Secondary Failures To Tolerate) and FTM (Failure Tolerance Method) in to account. If PFTT=1 and SFTT=2 and FTM=Erasure Coding (RAID5/6) then the minimum number of hosts per site is 6. You could however have 10 hosts in Site A while having 6 hosts in Site B.

Some may wonder why anyone would want this, well you can imagine you are running workloads which do not need to be recovered in the case of a full site failure. If that is the case you could stick these to 1 site. (You can set PFTT=0 and SFTT=1, which would result in this scenario for that particular VM.)

That is the power of Policy Based Management and vSAN, extremely flexibility and on a per VM/VMDK basis!

Related

Software Defined, Storage, vSAN 6.6, 6.6.1, VMware, vsan, vSphere

Reader Interactions

Comments

  1. jperformer says

    31 July, 2017 at 13:33

    Perfect tip !

  2. Totie Bash says

    24 August, 2017 at 04:54

    Hi Duncan, this question is VSAN related. I was surprise to see no change on VSAN configuration maximus “Virtual machines per host = 200”. For VDI use case this seem low especially with modern Intel CPU that could have 24+ core per processor? https://wojcieh.net/vsan-configuration-maximums/

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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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