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. The sense codes allow your host to recognize that the device has failed and register the state of the device as PDL. The sense codes must be received on all paths to the device for the device to be considered permanently lost.
After registering the PDL state of the device, the host stops attempts to reestablish connectivity or to issue commands to the device to avoid becoming blocked or unresponsive.
The vSphere Web 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 grayed out.
The host automatically removes the PDL device and all paths to the device if no open connections to the device exist, or after the last connection closes. You can disable the automatic removal of paths by setting the advanced host parameter Disk.AutoremoveOnPDL to 0. See Set Advanced Host Attributes.
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.
The host cannot detect PDL conditions and continues to treat the device connectivity problems as APD when a storage device permanently fails in a way that does not return appropriate SCSI sense codes or iSCSI login rejection.
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
For information about SCSI sense codes, see Troubleshooting Storage in vSphere Troubleshooting.
Permanent Device Loss and iSCSI
In the case of 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 terminates all I/O from virtual machines. vSphere HA can detect PDL and restart failed virtual machines. For more information, see Device Connectivity Problems and High Availability.