Home > Storage > PowerMax and VMAX > Storage Admin > Dell PowerMax and VMware vSphere Configuration Guide > All Paths Down (APD) and Permanent Device Loss (PDL)
All paths down or APD, and permanent device loss or PDL, are conditions that result from loss of access to a storage device. Depending on how the host loses access to the device, one of these two conditions will occur.
APD occurs on an ESXi host when a storage device is removed in an uncontrolled manner from the host (or the device fails), and the VMkernel core storage stack does not know how long the loss of device access will last. VMware, however, assumes the condition is temporary. A typical way of getting into APD would be if the zoning was removed.
PDL is similar to APD, and hence why VMware could not distinguish between the two in earlier vSphere releases, except it represents an unrecoverable loss of access to the storage. VMware assumes the storage is never coming back. Removing the device backing the datastore from the storage group would produce the error.
VMware’s approach to APD/PDL has evolved over the vSphere versions beginning in vSphere 4. Only the current implementation is covered here as the other versions are deprecated.
Note: Redundancy of a multi-pathing solution, such as PowerPath/VE, can help avoid some situations of APD.
Note: VMware relies on SCSI sense codes to detect PDL. If a device fails in a manner that does not return the proper sense codes, VMware will default to APD behavior.
The major concern with APD/PDL is that VMs running on those devices (datastores/RDMs) will be hung, causing unwanted outages. In cases where device removal is expected, steps can be taken to avoid these conditions. This will be covered below in Avoiding APD/PDL. But when it is unexpected, starting with vSphere 6, VMware offers some capabilities around APD and PDL for the HA cluster which allow automated recovery of VMs. The capabilities are enabled through a feature called VM Component Protection or VMCP. When VMCP is enabled, vSphere can detect datastore accessibility failures, APD or PDL, and then recover affected virtual machines. VMCP allows the user to determine the response that vSphere HA will make, ranging from the creation of event alarms to virtual machine restarts on other hosts.
VMCP is enabled automatically (Host Monitoring) once vSphere HA is enabled. The screen is shown in Figure 54 in the vSphere Client under the vSphere Availability menu option on the left.
Note: VMCP is not supported with vVols.
Once VMCP is enabled, storage protection levels and virtual machine remediations can be chosen for APD and PDL conditions as shown in Figure 55.
Each condition can be configured independently. The following sections will detail the best practices for each.