Tagged Queuing; Untagged Queuing; Command Queuing Rule; Queue Full Status - IBM DDRS-39130 - Ultrastar 9.1 GB Hard Drive Specifications

10.3.2 Tagged queuing

Commands with a tag message are saved in the command queue. Queued commands will be reordered by
the target defined rule. See the section on Reordering 10.4, "Command reordering" on page 189 for details.

10.3.3 Untagged queuing

The target supports queuing one I/O process from each initiator. If the target receives an untagged I/O
process while executing an I/O process from a different initiator, the untagged I/O process may be queued.
Untagged I/O processes are treated by the target as though they were received with Simple Queue Tag mes-
sages for purposes of queuing.
Note: There is no guarantee that I/O processes are executed in the order they were
received in a multiple initiator environment when Untagged Queuing is enabled.

10.3.4 Command queuing rule

Commands can be received during an active I/O process if Bus is free. If CPE (concurrent process enable)
bit of Mode page 0 is 0, only a single command except 2 commands (Inquiry and Request Sense) can be
executed at the same time. In that case, a command starts to be executed after the previous command has
completed. If CPE (concurrent process enable) bit of Mode page 0 is 1, I/O processes of Read(6), Read
extend(10), Write(6) and Write extend(10) can be active at the same time.
Process" on page 189 for details.

10.3.5 Queue Full status

This status is returned when a Simple Queue tag, Ordered Queue tag or Head of Queue tag message is
received and the command queue is full. The I/O process is not placed in the command queue. Since one
queue element is reserved for each initiator, any untagged command that doesn't cause Incorrect Initiator
Connection will not cause Queue Full status.

10.3.6 Device behaviour on Command queuing

1. Initiators must send a Queue tag immediately after the Identify message in Message Out phase just after
Selection. Targets send a simple queue tag immediately after the Identify message in Message In phase
just after Reselection.
2. Each initiator can issue either one of tagged command or untagged command exclusively at the same
time. There can exist other initiator using the other.
3. When DQue (Disable queue) of mode page 0Ah is 1, if an initiator issues a tagged command, the drive
returns "Message Reject" message (07h) and receives that command as an untagged command.
4. Queue Tag number doesn't affect the order to execute.
5. An initiator issues a command with a queue tag which is same as the current I/O process or queued I/O
process, the target returns Incorrect Initiator connection.
6. A series of linked commands are a single I/O process, and are assigned the queue tag established in the
initial selection. A command received with a Head-of-Queue tag message shall not suspend a series of
linked commands for which the target has begun execution.
7. If DQue is changed to 1 while queued commands exist, all queued commands for the all initiators will
be aborted. All future commands received, from any initiator, with a queue tag will be processed as
untagged commands, with a message reject message being returned immediately after the qtag is received
by the target.
188
O E M Spec. of DDRS-3xxxx
See 10.5, "Concurrent I/O