VMware vSphere ESXi 7.x
VMware vSphere ESXi 8.x
All-Paths-Down (APD) situation occurs when all paths to a device are down. As there is no indication of 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 un-presented from the ESXi host.
The timeout period begins when the storage device becomes unavailable to your ESXi host and enters the APD state. By default, the APD timeout is set to 140 seconds. While the timeout lasts, the host continues its attempts to reestablish connectivity with the device. When the timeout ends and the device does not recover, the host stops its attempts to retry any I/O that is not coming from virtual machines.
The reasons for an APD state can be, for example, a failed switch or a disconnected storage cable.
A storage device is considered to be in the APD state when it becomes unavailable to your ESXi host for an unspecified period of time. 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. For more information, see Handling Transient APD Conditions.