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The I T L nexus specifies the relationship so that the drive can always reconnect to the initiator to restore the
pointers for task as long as only one command per I T L nexus is issued. It is the responsibility of the initiator to
assure that only one command is issued at any time.
7.7.2
Tagged task queuing
Tagged task queuing allows a drive to accept multiple commands from the same or different initiators until the
drive's task queue is full. A new task may be initiated any time the BUS FREE phase exists, if the disconnect
privilege is granted. If the disconnect privilege is not granted for a tagged command, the drive returns Busy sta-
tus to the new task.
The Task Queue Tag messages (see Section 4.4) allow the initiator to establish a unique I T L Q nexus to iden-
tify each task. Each task may be a command or a set of linked commands with a unique queue tag.
The I T L Q nexus allows the target to reconnect to the desired task and the initiator to restore the correct set of
pointers. a SCSI initiator port may have several task ongoing to the same or different logical unit as long as
each has a unique nexus.
If only Simple Task Queue Tag messages are used, the drive may execute the commands in any order that is
deemed desirable within the constraints of the queue management algorithm specified in the Control Mode
page (see Seagate SCSI Command Reference Manual, Part number 100293068). The command ordering is
done by the drive to meet its performance and functional goals. The algorithm used by the drive attempts to
achieve certain drive or system performance goals established in the drive firmware for the queued commands
and guarantee that all commands will be executed. One possible goal would be to minimize seek times, but
there could be others, possibly designed to meet some special system need. Commands from other initiators
are also executed in an order selected in the same manner. The drive uses the Simple Task Queue Tag when
reconnecting to the initiator.
If Ordered Task Queue Tag messages are used, the drive executes the commands in the order received with
respect to other commands received with Ordered Task Queue Tag messages. All commands received with a
Simple Task Queue Tag message prior to a command received with an Ordered Task Queue Tag message,
regardless of initiator, are executed before that command with the Ordered Task Queue Tag message. All com-
mands received with a Simple Task Queue Tag message after a command received with an Ordered Task
Queue Tag message, regardless of initiator, are executed after that command with the Ordered Task Queue
Tag message.
A command received with a Head of Task Queue Tag message is placed first in the queue, to be executed
next. A command received with a Head of Task Queue Tag message does not suspend a task for which the
drive has begun execution. Consecutive commands received with Head of Task Queue Tag messages are
executed in a last-in-first-out order.
The Control Mode page specifies alternative queue management algorithms with additional rules on the order
of execution of commands (see Seagate SCSI Command Reference Manual, Part number 100293068).
A task received from a SCSI initiator port without a task queue tag message while there are any tagged I/O
commands in the command queue from that initiator is an incorrect initiator connection, unless there is a con-
tingent allegiance condition. A task received from a SCSI initiator port with a task queue tag message while
there is an untagged command in the command queue from that initiator is also an incorrect initiator connec-
tion. In either of these cases, the drive removes all commands in the queue from that initiator, aborts the com-
mand in process if it is from that initiator, and sets the Sense Key to Aborted Command and the Sense Code to
Overlapped Commands Attempted.
The RESERVE and RELEASE commands should be sent with an Ordered Task Queue Tag message. Use of
the Head of Task Queue Tag message with these commands could result in reservation conflicts with previ-
ously issued commands.
The TEST UNIT READY and INQUIRY commands are often sent with a Head of Task Queue Tag message,
since the information returned is either available or has no effect on the condition of the drive.
148
Parallel SCSI Interface Product Manual, Rev. A
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