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.