Non-VI workload detected on the datastore

VMware excel at providing vague error messages! I received the error message Non-VI workload detected on the datastore. 

This is a precautionary error message and can be safely ignored. VMware have a KB article describing the causes of it. The KB article lists the following situations that cause the error:

• The host is running in an unsupported configuration.
• The storage array is performing a system operation such as replication or RAID reconstruction.
• VMware Consolidated Backup or vStorage APIs for Data Protection are accessing a snapshot on the datastore for backup purposes.
• The storage media (spindles, SSD) on which this datastore is located is shared with volumes used by non-vSphere workloads

There’s one missing though: if you have a storage system that uses an RDM gatekeeper, you will receive this error message. An RDM gatekeeper LUNs are a special type of LUN used by storage systems to enable communication with a host (these are similar to Hitachi Command Devices). RDM gatekeeper LUNs are attached to virtual machines as Mapped Raw LUNs. An example of this is below.