Priority Commands; Command Queuing - IBM Ultrastar 36LZX Specifications

3. If the issuing Initiator is the one that made the reservation but is not the one to receive the reservation
then
Ÿ An Inquiry, Request Sense, Reserve, or Release command is permitted.
Ÿ Any other command results in a Reservation Conflict Status.
11. If the issuing Initiator is not the one that made the reservation but is the one to receive the
reservation
then
Ÿ A Reserve command results in a Reservation Conflict Status.
Ÿ A Release command is permitted but is ignored.
Ÿ Any other command is permitted.
If a Reservation Conflict Status is not reported and the command is permitted, then the Target checks
the next highest priority internal condition to determine whether execution is allowed. See 11.1.1,
"Priority
of SCSI Status Byte Reporting."

11.2 Priority commands

Certain SCSI commands always execute without returning a Busy Status or Reservation Conflict Status
in response to the command. These commands are
Ÿ Inquiry
Ÿ Request Sense
These commands do not disconnect from the SCSI bus prior to completion. They are executed prior to
attempting to complete the execution of any other pending command that has disconnected from the
SCSI bus. Therefore, a second priority command cannot be received during the execution of a priority
command.
These commands are never queued whether or not the command is sent with a queue tag. However, the
rule for an Incorrect Initiator Connection still apply to priority commands. See 11.1.3, "Incorrect Initiator
Connection."

11.3 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 and will be immediately executed without Bus disconnection:
Ÿ Priority Commands (i.e.: Request Sense and Inquiry)
Ÿ Commands linked to previous commands. These are defined to be part of a single I/O process.
(Linked commands are always executed immediately following the previous command from the
same initiator. No other Initiator's command are allowed to be executed between two linked com-
mands.)
Ÿ Commands for which disconnection is not allowed. (These may result in a Busy Status.)
Ÿ Commands in which a SCSI bus error occurred between selection and first disconnection following
the receipt of the CDB.
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