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.
I had this question last week around vSAN 2-node direct connect and whether using a crossover cable is still required to be used or if a regular CAT6 cable (CAT 5E works as well) can be used. I knew the answer and figured this would be documented somewhere, but it doesn’t appear to be. To be honest, many websites when talking about the need for crossover cables are blatantly wrong. And yes, I also spotted some incorrect recommendations in VMware’s own documentation, so I requested those entries to be updated. Just to be clear, with vSAN 2-Node Direct Connect, or vMotion, or any other service for that matter, you can use a regular CAT6 cable, combined with 10GbE (or faster) NICs, this gives you great performance without the cost of a 10GbE (or faster) switch. I can’t recall having seen a NIC in the past 10 years that does not have Auto MDI/MDI-X implemented, even though it was an optional feature in the 1000Base-T standard. In other words, there’s no need to buy a crossover cable, or make one, just use a regular cable.