vSAN implements a concept identified as fault domains. The fault domains feature allows vSAN to group together multiple hosts (typically within the same chassis or rack) into a logical boundary domain. The fault domains setting makes sure that multiple replica copies of storage objects are not saved within the same boundary, but are instead distributed across the domains. This way, in case of the failure of an entire domain (chassis or rack), only one replica is affected.

The following figure displays how a storage object of a virtual machine is saved within vSAN when the fault domains feature is enabled. In this case, three fault domains are identified and each one contains either one replica copy of the object (R1 or R2) or the witness (W).