vSAN Stretched Cluster failure matrix

Duncan Epping · May 30, 2023 · Leave a Comment

The last couple of weeks I was involved internally in a discussion around the different vSAN stretched cluster failure scenarios. I wrote a lengthy email about how vSAN and HA would respond in certain scenarios. I have documented many of these over the years on my blog already, but never really published them as a whole.

In some of the scenarios below, I discuss a “partition”, a partition is a scenario where both the L3 connection to the witness is down and the ISL to the other site for one of the locations. So in the diagram above for instance, if I say that Site B is partitioned then it means that Site A can still communicate with the witness, but Site B cannot communicate with the Witness and cannot communicate with Site A either.

For all of the below scenarios the following applies, Site A is the preferred location and Site B is the secondary location. When it comes to the table, the first two columns refer to the policy setting for the VM as shown in the screenshot below. The third column refers to the location where the VM runs from a compute perspective. The fourth discusses the type of failure, and the fifth and sixth columns discuss the behavior witnessed.

Time to list the various scenarios, and no, it doesn’t include all failures that could occur, but should discuss most which are important for a stretched cluster configuration. Do note, below discussed behavior will only be witness when the best practices, as documented here and here, are followed. Also note that the table has multiple pages, there are close to 30 scenarios described! If there are any questions feel free to leave a comment, if you feel a failure scenario is missing, also please leave a comment.