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Nested Fault Domains on a 2-Node vSAN Stretched Cluster, is it supported?

Duncan Epping · Jun 20, 2022 · 2 Comments

I spotted a question this week on VMTN, the question was fairly basic, are nested fault domains supported on a 2-node vSAN Stretched Cluster? It sounds basic, but unfortunately, it is not documented anywhere, probably because stretched 2-node configurations are not very common. For those who don’t know, with a nested fault domain on a two-node cluster you basically provide an additional layer of resiliency by replicating an object within a host as well. A VM Storage Policy for a configuration like that will look as follows.

This however does mean that you would need to have a minimum of 3 fault domains within your host as well if you want to, this means that you will need to have a minimum of 3 disk groups in each of the two hosts as well. Or better said, when you configure Host Mirroring and then select the second option failures to tolerate the following list will show you the number of disk groups per host you need at a minimum:

  • Host Mirroring – 2 Node Cluster
    • No Data Redundancy – 1 disk group
    • 1 Failure – RAID1 – 3 disk groups
    • 1 Failure – RAID5 – 4 disk groups
    • 2 Failures – RAID1 – 5 disk groups
    • 2 Failures – RAID6 – 6 disk groups
    • 3 Failures – RAID1 – 7 disk groups

If you look at the list, you can imagine that if you need additional resiliency it will definitely come at a cost. But anyway, back to the question, is it supported when your 2-node configuration happens to be stretched across locations, and the answer is yes, VMware supports this.

Related

BC-DR, Storage, vSAN 2-node, stretched, VMware, vsan, vSphere

Reader Interactions

Comments

  1. Andrea Scarabelli says

    19 July, 2022 at 11:35

    Thanks Duncan for this post! Indeed long overdue.

    But what about the licensing? I just spotted in the updated official vSAN licensing guide that “Stretched
    cluster with local failure protection” is available also in the vSAN STD edition if limited to two-node cluster.

    Do you think that this means I can create a “two node stretched cluster with Nested Fault Domains” starting from vSAN STD?

    Reply
    • Duncan Epping says

      19 July, 2022 at 13:04

      Yes it works, but has not been explicitly tested for 2-node stretched across + 1ms latency.

      Reply

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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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