A storage device is considered to be in the all paths down (APD) state when it becomes unavailable to your ESXi host for an unspecified time period.
The reasons for an APD state can be, for example, a failed switch or a disconnected storage cable.
In contrast with the permanent device loss (PDL) state, the host treats the APD state as transient and expects the device to be available again.
The host continues to retry issued commands in an attempt to reestablish connectivity with the device. If the host's commands fail the retries for a prolonged period, the host might be at risk of having performance problems. Potentially, the host and its virtual machines might become unresponsive.
To avoid these problems, your host uses a default APD handling feature. When a device enters the APD state, the host turns on a timer. With the timer on, the host continues to retry non-virtual machine commands for a limited time period only.
By default, the APD timeout is set to 140 seconds. This value is typically longer than most devices require to recover from a connection loss. If the device becomes available within this time, the host and its virtual machine continue to run without experiencing any problems.
If the device does not recover and the timeout ends, the host stops its attempts at retries and stops any non-virtual machine I/O. Virtual machine I/O continues retrying. The vSphere Client displays the following information for the device with the expired APD timeout:
The operational state of the device changes to Dead or Error.
All paths are shown as Dead.
Datastores on the device are dimmed.
Even though the device and datastores are unavailable, virtual machines remain responsive. You can power off the virtual machines or migrate them to a different datastore or host.
If later the device paths become operational, the host can resume I/O to the device and end the special APD treatment.