vSAN fault domains provide the ability to tolerate rack failures and power failures in addition to disk, network, and host failures.

The idea behind fault domains is to tolerate groups of hosts (chassis or racks) failing without requiring additional data copies. Their implementation allows vSAN to save replica copies of the virtual machine data in different domains, for example, different racks of compute. An example of using fault domains is shown in the following figure.

 

 

When working with fault domains, to tolerate n number of failures, rather than needing 2n+1 hosts, you require 2n+1 fault domains instead. This allows for replica data to be spread out across the domains rather than hosts and increases the ability to handle a greater than single host failure.

A minimum of three fault domains are required, however, VMware recommends having four or more, for redundancy purposes. In addition, to have a minimum level of redundancy, VMware also recommends having six hosts (two per fault domain).

Fault domains are used to further protect the environment from failure, assuming there are enough hosts to properly support the configuration. Further details on fault domains can be found in the VMware Virtual SAN Design and Sizing Guide (https://www.vmware.com/files/pdf/products/vsan/VSAN_Design_and_Sizing_Guide.pdf).