Queue Depth; Tagged Queuing; Untagged Queuing; Command Queuing Rule - IBM Ultrastar 36LZX Specifications

Ÿ Commands for an invalid LUN.
Ÿ Commands which cause an OVERLAPPED COMMANDS ATTEMPTED error (see 11.1.3, "Incorrect
Initiator Connection").

11.3.1 Queue depth

Any initiator can queue at least one command at any time irrespective of the actions of any other initi-
ators in the system. A single initiator may queue up to 128 commands, if no other initiator has more than
one command in the queue, although at times this maximum may be reduced as the drive can reserve
command blocks for internal use.

11.3.2 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 11.4 "Command reordering" for details.

11.3.3 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
messages 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.

11.3.4 Command queuing rule

Commands can be received during an active I/O process if the 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.
See Concurrent I/O Process on page 239 for details.

11.3.5 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 does not cause Incorrect
Initiator Connection will not cause Queue Full status.

11.3.6 Device behavior 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 a tagged command or an untagged command exclusively at the
same time. Other initiators can exist which operate mutually exclusively with tagged or untagged
commands
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 com-
mand.
Ultrastar 36LZX hard disk drive specifications
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