Many block devices have a unique devid that identifies the particular physical media; e.g., a specific hard disk, or a specific logical volume. Unfortunately not every block device is required to provide one but ZFS assumes in some places that one will be available.

Of particular note is zpool_vdev_name(), used in zpool status to get the short /dev name of a disk vdev; e.g., /dev/dsk/c0t0d0 would be printed as c0t0d0. In addition to rendering the short name, this function is responsible for confirming that the /dev path is still correct. The path may have changed for devices (e.g., SATA disks) which have a /dev path based on their physical attachment point rather than the media identity. The zpool_vdev_name() function uses the devid value to determine an appropriate /dev path, and if it has changed it notifies the kernel that the ZFS pool configuration needs to be updated as well.

If the vdev does not have a devid then we do not update the /dev path at all, potentially leaving an incorrect value. This prevents processes that use the /dev path (e.g., installboot) from correctly interacting with the devices in the pool, and will almost certainly confuse the operator.

If a device does not have a devid, we should fall back to determining a valid /dev path for the physical path (i.e., the /devices path). If a vdev does not have a physical path either, we'll continue to do nothing for now.