Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
69
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If ORDERED QUEUE TAG messages are used, the disc drive executes the commands in the order received
with respect to other commands received with ORDERED QUEUE TAG messages. All commands received
with a SIMPLE QUEUE TAG message prior to a command received with an ORDERED QUEUE TAG mes-
sage, regardless of initiator, are executed before that command with the ORDERED QUEUE TAG message.
All commands received with a SIMPLE QUEUE TAG message after a command received with an ORDERED
QUEUE TAG message, regardless of initiator, are executed after that command with the ORDERED QUEUE
TAG message.
A command received with a HEAD OF QUEUE TAG message is placed first in the queue, to be executed
next. A command received with a HEAD OF QUEUE TAG message does not suspend an I/O process for
which the disc drive has begun execution. Consecutive commands received with HEAD OF QUEUE TAG
messages are executed in a last-in-first-out order.
The control mode page specifies alternative queue management algorithms with additional rules on the order
of execution of commands (see 5.2.1-28).
An I/O process received from an initiator without a queue tag message while there are any tagged I/O
commands in the command queue from that initiator is an incorrect initiator connection, unless there is a
contingent allegiance condition. An I/O process received from an initiator with a queue tag message while
there is an untagged command in the command queue from that initiator is also an incorrect initiator connec-
tion. In either of these cases the disc drive removes all commands in the queue from that initiator, aborts the
command in process if it is from that initiator, and sets the Sense Key to Aborted Command and the Sense
Code to Overlapped Commands Attempted.
The RESERVE and RELEASE commands should be sent with an ORDERED QUEUE TAG message. Use of
the HEAD OF QUEUE TAG message with these commands could result in reservation conflicts with previ-
ously issued commands.
The TEST UNIT READY and INQUIRY commands are often sent with a HEAD OF QUEUE TAG message,
since the information returned is either available or has no effect on the condition of the disc drive.
The disc drive recovery option, is to continue execution of commands in the queue after the contingent
allegiance condition has cleared. The disc drive returns BUSY status to all other initiators while the contin-
gent allegiance condition exists. During this time all commands in the queue are suspended. All commands
used for recovery operations are untagged commands.
Deferred errors are normally related to a command that has already completed. As such, there is no attempt
to return the queue tag value assigned to the original command.
