Command Processing Considerations And Exception Conditions; Auto Contingent Allegiance Or Contingent Allegiance; Logical Unit Response To Auto Contingent Allegiance Or Contingent Allegiance - Seagate SCSI Interface Product Manual

66
4.6

Command processing considerations and exception conditions

The following clauses describe some exception conditions and errors associated with command processing
and the sequencing of commands.
4.6.1

Auto Contingent Allegiance or Contingent Allegiance

The auto contingent allegiance (NACA=1, see Section 4.2.6) or contingent allegiance (NACA=0) condition
shall exist within the task set when the logical unit completes a command by returning a Check Condition sta-
tus (see Section 4.3).
4.6.1.1

Logical Unit response to Auto Contingent Allegiance or Contingent Allegiance

The auto contingent allegiance (NACA=1, see Section 4.2.6) or contingent allegiance (NACA=0) condition
shall not cross task set boundaries and shall be preserved until it is cleared as described in Section 4.6.1.2. If
requested by the application client and supported by the protocol and logical unit, sense data shall be returned
as described in Section 4.6.4.2.
Notes.
1. The SCSI-2 contingent allegiance condition has had an alternate added and the extended contingent alle-
giance condition has been replaced in SCSI-3 by auto contingent allegiance in conjunction with the NACA
bit.
2. If the SCSI-3 protocol does not enforce synchronization of client/server states, there may be a time delay
between the occurrence of the auto contingent allegiance or contingent allegiance condition and the time
at which the initiator becomes aware of the condition.
After sending status and a service response of Task Complete, the logical unit shall modify the state of all tasks
in the faulted task set as described in ANSI document T10/1157D, clause 7, "Task Set Management."
A task created by the faulted initiator while the auto contingent allegiance condition is in effect may be entered
into the faulted task set under the conditions described below.
As described in Section 4.6.1.2, the setting of the NACA bit in the Control byte of the faulting command CDB
determines the rules that apply to an ACA or CA condition caused by that command. If the NACA bit was set to
zero the SCSI-2 contingent allegiance rules shall apply.
If the NACA bit was set to one in the Control byte of the faulting command, then a new task created by the
faulted initiator while the ACA condition is in effect shall not be entered into the faulted task set unless all of the
following conditions are true:
a. The task has the ACA attribute; and
b. No other task from the faulted initiator having the ACA attribute is in the task set.
If the task is from the faulted initiator and any of the conditions listed above are not met, the newly created task
shall not be entered into the task set and shall be completed with a status of ACA Active.
If a task having the ACA attribute is received and no auto contingent allegiance condition is in effect for the task
set or if the NACA bit was set to zero in the CDB for the faulting command (i.e., a contingent allegiance condi-
tion is in effect), then the ACA task shall be completed with a status of Check Condition. The sense key shall be
set to Illegal Request with an additional sense code of Invalid Message Error. As noted in Section 4.6.1.2, any
existing contingent allegiance condition shall be cleared and a new auto contingent allegiance (NACA=1) or
contingent allegiance (NACA=0) condition shall be established.
The handling of tasks created by initiators other than the faulted initiator depends on the value in the TST field
in the Control mode page (see ANSI document SPC-2, T10/1236-D).
If TST=000b, tasks created by other initiators while the ACA or CA condition is in effect shall not be entered
into the faulted task set (except for a Persistent Reserve command with a Preempt and Clear action as
described in Section 4.6.1.2). Tasks rejected from the task set due to the presence of an ACA or CA condition
shall be completed with a status of ACA Active (if NACA=1 in the new command's CDB Control byte, see Sec-
tion 4.2.6) or Busy (if NACA=0).
If TST=001b, tasks created by one initiator shall not be rejected based on an ACA or CA condition in effect for
another initiator. Only ACA or CA condition for the sending initiator (as well as other task set management con-
SCSI Interface Product Manual, Rev. J