When your ESXi host experiences a problem while connecting to a storage device, the host treats the problem as permanent or temporary depending on certain factors.
Storage connectivity problems are caused by a variety of reasons. Although ESXi cannot always determine the reason for a storage device or its paths being unavailable, the host differentiates between a permanent device loss (PDL) state of the device and a transient all paths down (APD) state of storage.
Permanent Device Loss (PDL)
A condition that occurs when a storage device permanently fails or is administratively removed or excluded. It is not expected to become available. When the device becomes permanently unavailable, ESXi receives appropriate sense codes or a login rejection from storage arrays, and is able to recognize that the device is permanently lost.