Cross-Site Witness Support for vSAN Stretched clusters
Can two stretched clusters be hosting each other’s witness hosts? We get that question often, and the short answer is – yes, but only if you have four independent sites. Let’s look what is the reason behind this requirement.
Let’s say we have two different stretched clusters (stretched cluster ‘A’ and stretched cluster ‘B’). Now, can we have the witness for stretched cluster ‘A’ deployed on stretched cluster ‘B’ and vice-versa? Below you can find more details on the supported configuration for cross-site witness support, and the reasons why we do not support that configuration with only 2 sites.
- Cross-Site Witness Support is not supported on two sites hosting two stretched cluster implementations
Setting up cross witnesses for two stretched clusters on only two sites is not supported. The reason why we do not support this configuration is that each stretched cluster has a dependency on a site and might result in more than one site failure.
Let’s examine the following configurations, where we have two sites and two stretched clusters.
In this scenario, we have two sites, and both stretched cluster A and stretched cluster B are stretched across Site 1 and Site 2. Stretched cluster A (SCA) hosts Witness B(WB) and Stretched cluster B (SCB) hosts Witness A(WA). In this case when one of the sites fails this might results in more than one site failure for both stretched cluster topologies. The VM residing on SCA will become unavailable due to losing one site and the witness site. While SCB will lose one site. This cross-witness support configuration will result in the same outcome if the alternate site fails.
Of course, we could have both witnesses on one site and lose only the site that doesn’t host any witnesses. In this case, there will be a site failure for each stretched cluster, but no impact on the VM’s running state.
In this same configuration, losing Site 1 will impact the VMs’s running state of both SCA and SCB since both of them will experience more than one site failure.
2. Cross-Site Witness Support supported on four sites hosting two stretched cluster implementations
Let’s have a look at what are the improvements if we have four unique data sites. SCA is implemented across two sites and stretched cluster B is implemented across two other sites. There are no dependencies between any of the sites, other than the fact that the witness appliance is hosted on the other stretched cluster, WA on SCB, and WB on SCA. One site failure will not affect the object’s quorum and the VM will be up and running.
For more details on the Stretched cluster configurations, please take a look at our “vSAN Stretched cluster guide”. Also, don’t forget to check our “vSAN Interactive infographic” where you can find the answers on how vSAN handles different failure scenarios.