Working with Permanent Device Loss
With earlier ESX/ESXi releases, an APD (All Paths Down) event results when the LUN becomes unavailable. The event is difficult for administrators because they do not have enough information about the state of the LUN to know which corrective action is appropriate.
In ESXi 5.0, the ESXi host can determine whether the cause of an All Paths Down (APD) event is temporary, or whether the cause is permanent device loss. A PDL status occurs when the storage array returns SCSI sense codes indicating that the LUN is no longer available or that a severe, unrecoverable hardware problem exists with it. ESXi has an improved infrastructure that can speed up operations of upper-layer applications in a device loss scenario.
To Remove a PDL LUN
How you remove a PDL LUN depends on whether it was in use.
To Reattach a PDL LUN
You cannot bring a device back without removing active users. The ESXi host cannot know whether the device that was added back has changed. ESXi must be able to treat the device similarly to a new device being discovered.