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All paths down (APD) and permanent device loss (PDL) are conditions that result from loss of access to a storage device. Depending on how the host loses access to a device, one of these two conditions occurs.
APD occurs on an ESXi host when both:
VMware assumes that the condition is temporary. A typical way of getting into APD would be to remove the zoning.
PDL is similar to APD. VMware could not distinguish between the two in earlier vSphere releases, except it represents an unrecoverable loss of access to the storage. VMware assumes that the storage is never coming back. Removing the device backing the datastore from the storage group would produce the error.
The approach that VMware takes to APD and 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 multipathing solution, such as PP/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 defaults to APD behavior.
The major concern with APD and PDL is that VMs running on those devices (datastores/RDMs) can stop responding, causing unwanted outages. In cases where device removal is expected, steps can be taken to avoid these conditions. This procedure is covered below in Avoiding APD and PDL. But when it is unexpected, 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 that is 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 makes, 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 detail the best practices for each.