Priority Commands; Command Queuing; Queue Depth - IBM DDRS-39130 - Ultrastar 9.1 GB Hard Drive Specifications

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 10.1.1, "Priority of
SCSI Status Byte Reporting" on page 179

10.2 Priority Commands

Certain SCSI commands always execute without returning a Busy Status, Reservation Conflict Status in
response to the command. These commands are:
Inquiry
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 10.1.3, "Incorrect Initiator
Connection" on page 180)

10.3 Command queuing

When the initiator specifies that the file shall disable command queuing, the initiator must send only
untagged command. 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
Priority Commands(i.e.: Request Sense and Inquiry)
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 commands.)
Commands for which disconnection is not allowed.(These may result in
a Busy Status.)
Commands in which a SCSI bus error occurred between selection and
first disconnection following the receipt of the CDB.
Commands for an invalid LUN.
Commands which cause an OVERLAPPED COMMANDS A T TE MPT ED
error (see 10.1.3, "Incorrect Initiator Connection" on page 180).

10.3.1 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 single initiator may queue up to 64 commands, if no other initiator has more than one
command in the queue, although at times this maximum may be reduced as the file can reserve command
blocks for internal use.
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