A storage device is considered to be in the permanent device loss (PDL) state when it becomes permanently unavailable to your ESXi host.

Typically, the PDL condition occurs when a device is unintentionally removed, or its unique ID changes, or when the device experiences an unrecoverable hardware error.

When the storage array determines that the device is permanently unavailable, it sends SCSI sense codes to the ESXi host. After receiving the sense codes, your host recognizes the device as failed and registers the state of the device as PDL. For the device to be considered permanently lost, the sense codes must be received on all its paths.

After registering the PDL state of the device, the host stops attempts to reestablish connectivity or to send commands to the device.

The vSphere Client displays the following information for the device:
  • The operational state of the device changes to Lost Communication.
  • All paths are shown as Dead.
  • Datastores on the device are not available.

If no open connections to the device exist, or after the last connection closes, the host removes the PDL device and all paths to the device. You can disable the automatic removal of paths by setting the advanced host parameter Disk.AutoremoveOnPDL to 0.

If the device returns from the PDL condition, the host can discover it, but treats it as a new device. Data consistency for virtual machines on the recovered device is not guaranteed.

Note: When a device fails without sending appropriate SCSI sense codes or an iSCSI login rejection, the host cannot detect PDL conditions. In this case, the host continues to treat the device connectivity problems as APD even when the device fails permanently.

Permanent Device Loss and SCSI Sense Codes

The following VMkernel log example of a SCSI sense code indicates that the device is in the PDL state. 
H:0x0 D:0x2 P:0x0 Valid sense data: 0x5 0x25 0x0 or Logical Unit Not Supported

Permanent Device Loss and iSCSI

On iSCSI arrays with a single LUN per target, PDL is detected through iSCSI login failure. An iSCSI storage array rejects your host's attempts to start an iSCSI session with a reason Target Unavailable. As with the sense codes, this response must be received on all paths for the device to be considered permanently lost.

Permanent Device Loss and Virtual Machines

After registering the PDL state of the device, the host closes all I/O from virtual machines. vSphere HA can detect PDL and restart failed virtual machines.