Powering on or migrating a virtual machine fails with the error: Virtual Disk ‘X’ is a mapped direct access LUN that is not accessible. This is an informational event which indicates that the host attempting to start or receive the virtual machine cannot access the device (LUN) backing the raw disk mapping.
A raw device mapping file contains metadata, specifically SCSI vital product data (VPD) page 0x83, for uniquely identifying a device. A host attempting to access the physical LUN reads the mapping file metadata, and searches the devices known to the host for a LUN with the same VPD page 0x83 information.
The contents of SCSI VPD page 0x83 should match when requested by all hosts in a vSphere cluster, and the RDM should be accessible by all hosts in the cluster. However, discrepancies in device presentation between hosts in a cluster may lead to different hosts receiving different contents for VPD page 0x83 for the same device. In this case, hosts which did not create the RDM mapping file may be unable to locate the raw device mapping LUN when powering on or migrating a virtual machine.
For a complete resolution read the following Vmware knowledge base article.
Posted in: VMware vSphere