Hitachi Ultrastar 15K450 (FC-AL) Hard Disk Drive Specification
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21.8 Command Queuing
When the initiator specifies that the drive shall disable command queuing, the initiator must send only untagged commands.
When the initiator specifies that the target shall enable command queuing, the initiator may send either tagged or untagged
command, but shall not use both at the same time.
The following commands are never queued.
• Priority Commands (i.e.: Request Sense and Inquiry)
• Commands for an invalid LUN.

21.8.1 Queue Depth

Any initiator can queue at least one command at any time irrespective of the actions of any other initiators in the system. A sin-
gle 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.

21.8.2 Queue Full Status

The drive will respond with QUEUE FULL status to a SCSI command when all queue slots are utilitzed. The SCSI command
is not placed in the command queue under this condition.

21.8.3 Effects of LIP on Command Queuing

When a loop re-initializes (via a LIP sequence) command execution for all initiators on the initializing port is suspended. After
loop initialization is complete, the drive will maintain all queued commands for all initiators up to RR_TOV. If an initiator per-
forms Exchange Authentication (via PDISC or ADISC) within RR_TOV, its queued commands will now be free to execute.
If an initiator fails to authenticate within RR_TOV, the initiator will be implicitly logged out, and the commands will be
removed from the command queue.

21.8.4 Termination of I/O Processes

Normal termination of I/O processes occurs when the target returns SCSI status. I/O processes may also be terminated by the
following:
• An ABORT TASK terminates the specified I/O process from the issuing initator
• An ABORT TASK SET terminates all I/O processes from the issuing initiator
• A CLEAR TASK SET, TARGET RESET or reset terminates all I/O processes from all initiators
• Failure by an initiator to initiate Exchange Authentication within RR_TOV of the completion of Loop Initialization
results in an implicit logout and termination of all associated I/O processes (21.8.5 Effects of LIP on Command Que-
ing on a page 291)
21.9 Command Reordering
Command reordering is supported when enabled by the Queue Algorithm Modifier in mode page 0A ( see 19.10.9, “Mode
Page 0A (Control Mode Page Parameters)” on page 186.
21.10 Concurrent I/O Process
Concurrent command are always allowed to execute concurrently with non-priority commands. A second priority command
received while a priority command is being executed is put at the head of the command queue.
• WRITE commands when another WRITE command is an active I/O process
• READ commands when another READ command is an active I/O process
When a concurrent command ends in CHECK CONDITION status, the QErr bit on the Mode Page 0Ah will determine how
other active I/O processes from the same initiator for that drive will be handled.