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• If disconnect processing is impossible.
If disconnect processing is impossible because the INIT which issued the command does not
meet the conditions for permitting a disconnect (see item 1.3.3), or if the DISCONNECT
message is rejected by the INIT even though it meets the conditions for permitting a
disconnect, when another command is already being executed, or if another command is
already being executed or queuing is currently being executed, or when the initial self-
diagnosis is being executed, except in cases covered in 1), the IDD responds with a BUSY
status without queuing the received command. If this is not the case, the received command is
executed immediately.
• If the IDD is reserved
If the IDD has been reserved by an INIT using the RESERVE command, and receives a TEST
UNIT READY, REQUEST SENSE or INQUIRY command after that, when that command
conflicts with the reserved state, it responds with a RESERVATION CONFLICT status.
Commands after that are queued, and the reserved state is checked when a command is fetched
from the queue.
RESERVE command (item 3.1.11).
Note
Through the operation of the command queuing function, except for exceptions described on
this page, the IDD does not respond to commands issued by the INIT with a BUSY status.
This function is applied under the multi-initiator environment, and overhead for re-issuing
commands caused by the BUSY status is unnecessary. Normally, the INIT does not have to
be aware of the existence of a queuing function, but it is necessary to exercise caution in the
following items when controlling input/output processing.
1) When a command is queued, the time from the queuing of the command to its actual
execution will vary depending on the commands already in the queue, or on the content of
the processing currently being executed. At times when a command is queued, the time
until that queued command is actually executed will vary depending on the commands
already in the queue ahead of that command, and the contents of processing of that
command when it is executed. Particularly in cases where the FORMAT UNIT command
and START/STOP UNIT command (Immed = 0), and data access commands which
specify large processing block counts, are already queued or being executed, the newly
queued command will be forced to wait a long time until it is executed.
2) In the following cases, a command may not be executed even after it has been queued.
a)
When there is an error in the CDB, the IDD responds with a CHECK CONDITION
status at the point when that command is fetched from the queue.
b)
If the IDD is in the not ready state at the point when the queued command is fetched,
it responds with a CHECK CONDITION status.
c)
If a UNIT ATTENTION condition is generated before the queued command is
fetched, it responds with a CHECK CONDITION status.before the command is
dequeued, a CHECK CONDITION status may be replied.
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Conflicts with the reserved state are explained in the description of the
C141-E039-01EN
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