If 'online' is 0 - the device was marked as offline due to some errors,
and any SCSI commands sent to it are failed by the Linux kernel.
'busy' shows the number of pending commands. An idle device with a 'busy'
field of more then 0 implies stuck SCSI commands.
'qdepth' is the number of commands that may be pending between the HBA
driver and the target. NOTE: there is still another queue of
commands that were not yet handled by the HBA, which can be much larger.