Backups fail as a result of a vSAN cluster node being offline.
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.
VMware recommends having at least 4 nodes for extra fault tolerance in the event that a node would unexpectedly go down.
For more details, please see: https://kb.vmware.com/s/article/2106708
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