Support for Queuing
For a definition of tagged queuing, refer to the SCSI-2 specification. To support tagged
queuing, first check the scsi_options flag SCSI_OPTIONS_TAG to see whether tagged queuing is enabled
globally. Next, check to see whether the target is a SCSI-2 device and
whether the target has tagged queuing enabled. If these conditions are all true,
attempt to enable tagged queuing by using scsi_ifsetcap(9F).
If tagged queuing fails, you can attempt to set untagged queuing. In this mode, you
submit as many commands as you think necessary or optimal to the host
adapter driver. Then the host adapter queues the commands to the target one
command at a time, in contrast to tagged queuing. In tagged queuing,
the host adapter submits as many commands as possible until the target
indicates that the queue is full.