Backup job fails with Creating VM snapshot Error: There are currently 2 usable fault domains. The operation requires 1 more usable fault domains when backing up from vSAN cluster

Challenge

Backups fail as a result of a vSAN cluster node being offline.

Cause

If the vSAN has fewer than 4 nodes and one of the nodes is offline, either for schedule maintenance or unexpectedly, snapshots cannot be taken of VMs on the vSAN cluster.

Solution

VMware recommends having at least 4 nodes for extra fault tolerance in the event that a node would unexpectedly go down.

More Information

This can occur with clusters of more than 3 nodes if enough nodes are offline to take the cluster down to 2 or fewer online nodes.

For more details, please see: https://pubs.vmware.com/vsphere-60/index.jsp?topic=%2Fcom.vmware.vsphere.virtualsan.doc%2FGUID-FE7DBC6F-C204-4137-827F-7E04FE88D968.html