Permanent Device Loss (PDL) and All-Paths-Down (APD)

In vSphere 4.x, an All-Paths-Down (APD) situation occurs when all paths to a device are down. As there is no indication whether this is a permanent or temporary device loss, the ESXi host keeps reattempting to establish connectivity. APD-style situations commonly occur when the LUN is incorrectly unpresented from the ESXi/ESX host. The ESXi/ESX host, still believing the device is available, retries all SCSI commands indefinitely. This has an impact on the management agents, as their commands are not responded to until the device is again accessible. This causes the ESXi/ESX host to become inaccessible/not-responding in vCenter Server.

Permanent Device Loss (PDL):

In vSphere 5.x, a clear distinction has been made between a device that is permanently lost (PDL) and a transient issue where all paths are down (APD) for an unknown reason.

For example, in the VMkernel logs, if a SCSI sense code of H:0x0 D:0x2 P:0x0 Valid sense data: 0x5 0x25 0x0 or Logical Unit Not Supported is logged by the storage device to the ESXi 5.xhost, this indicates that the device is permanently inaccessible to the ESXi host, or is in a Permanent Device Loss (PDL) state. The ESXi host no longer attempts to re-establish connectivity or issue commands to the device.

Devices that suffer a non-recoverable hardware error are also recognized as being in a Permanent Device Loss (PDL) state.

This table outlines possible SCSI sense codes that determine if a device is in a PDL state:

SCSI sense code	Description
H:0x0 D:0x2 P:0x0 Valid sense data: 0x5 0x25 0x0	LOGICAL UNIT NOT SUPPORTED
H:0x0 D:0x2 P:0x0 Valid sense data: 0x4 0x4c 0x0	LOGICAL UNIT FAILED SELF-CONFIGURATION
H:0x0 D:0x2 P:0x0 Valid sense data: 0x4 0x3e 0x3	LOGICAL UNIT FAILED SELF-TEST
H:0x0 D:0x2 P:0x0 Valid sense data: 0x4 0x3e 0x1	LOGICAL UNIT FAILURE

Note: Some iSCSI arrays map LUN-to-Target as a one-to-one relationship. That is, there is only ever a single LUN per Target. In this case, the iSCSI arrays do not return the appropriate SCSI sense code, so PDL on these arrays types cannot be detected. However, in ESXi 5.1, enhancements have been made and now the iSCSI initiator attempts to re-login to the target after a dropped session. If the device is not accessible, the storage system rejects the host's effort to access the storage. Depending on the response from the array, the host can now mark the device as PDL.

All-Paths-Down (APD)

If PDL SCSI sense codes are not returned from a device (when unable to contact the storage array, or with a storage array that does not return the supported PDL SCSI codes), then the device is in an All-Paths-Down (APD) state, and the ESXi host continues to send I/O requests until the host receives a response.

As the ESXi host is not able to determine if the device loss is permanent (PDL) or transient (APD), it indefinitely retries SCSI I/O, including:

·    Userworld I/O (hostd management agent)

·    Virtual machine guest I/O

Note: If an I/O request is issued from a guest, the operating system should timeout and abort the I/O.

Due to the nature of an APD situation, there is no clean way to recover.

·    The APD situation needs to be resolved at the storage array/fabric layer to restore connectivity to the host.

·    All affected ESXi hosts may require a reboot to remove any residual references to the affected devices that are in an APD state.

Note: Performing a vMotion migration of unaffected virtual machines is not possible, as the management agents may be affected by the APD condition, and the ESXi host may become unmanaged. As a result, a reboot of an affected ESXi host forces an outage to all non-affected virtual machines on that host.

Planned versus unplanned PDL

A planned PDL occurs when there is an intent to remove a device presented to the ESXi host. The datastore must first be unmounted, then the device detached before the storage device can be unpresented at the storage array. 

An unplanned PDL occurs when the storage device is unexpectedly unpresented from the storage array without the unmount and detach being executed on the ESXi host.

In ESXi 5.5, VMware provides a feature called Auto-remove for automatic removal of devices during an unplanned PDL.