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Does vSAN Enhanced Durability work when you have a limited number of hosts?

Duncan Epping · Apr 19, 2021 ·

Last week I had a question about how vSAN Enhanced Durability works when you have a limited number of hosts. In this case, the customer had a 3+3+1 stretched cluster configuration, and they wondered what would happen when they would place a host into maintenance mode. Although I was pretty sure I knew what would happen, I figured I would test it in the lab anyway. Let’s start with a high-level diagram of what the environment looks like. Note I use a single VM as an example, just to keep the scenario easy to follow.

In the diagram, we see a virtual disk that is configured to be stretched across locations, and protected by RAID-1 within each location. As a result, you will have two RAID-1 trees each with two components and a witness, and of course, you would have a witness component in the witness location. Now the question is, what happens when you place esxi-host-1 into maintenance mode? In this scenario, vSAN Enhanced Durability will want to create a “durability component”. This durability component is used to commit all new write IO to. This will allow vSAN to resync fast after maintenance mode, and enhances durability as we would still have 2 copies of the (new) data.

However, in the scenario above we only have 3 hosts per location. The question then is, where is this delta component created then? As normally with maintenance mode you would need a 4th host to move data to. Well, it is simple, in this case, what vSAN does is it creates a “durability component” on the host where the witness resides, within the location of course. Let me show you in a diagram, as that makes it clear instantly.

By adding the Durability component next to the witness on esxi-host-3, vSAN enhances durability even in this stretched cluster situation, as it provides a local additional copy of new writes. Now, of course I tested this in my lab. So for those who prefer to see a demo, check out the youtube video below.

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BC-DR, cloud, Data Recovery, Software Defined, Storage, VMware Cloud Foundation, vSAN 7.0, 7.0 u1, 7.0 u2, durability, durability components, enhanced durability, stretched cluster, VMware, vsan

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Comments

  1. Did says

    23 April, 2021 at 19:43

    Does that mean you need 10 Gb connectivity to the witness site?

    • Duncan Epping says

      23 April, 2021 at 20:11

      No you don’t. The bandwidth requirements to the witness site are low. What we are talking about here is the witness component in a data site!

  2. Andrea Mazzai says

    2 July, 2021 at 16:19

    Hi Duncan, in a three-node cluster (policy = Mirror) what happens if I have 2 DGs and two hosts are no more available (in example first host in EMM and second host with hardware failure)? Is the alive host able to continue operations for the VMs running on it whose one of the two components is residing in that same host itself? If yes, will vSAN try to create a durability component in the other disk group of the host itself?

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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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