Deferred Error vs. Unit Attention

Gerry_Houlder at Gerry_Houlder at
Tue Dec 15 07:30:47 PST 1998

* From the T10 Reflector (t10 at, posted by:
* Gerry_Houlder at
The deferred error reporting mechanism is intended for reporting an error
with a command that had previously been ended with GOOD status. It is not
used for any other situation. An example of the deferred error case:
(a) Initiator sends a WRITE command with Immediate bit set (tape devices do
this) or with WCE active on mode page Ah (direct access devices). The
target will take the data and return GOOD status right away.
(b) Later the target encounters an error while writing the data. It stops
command execution and prepares a deferred error report.
(c) The target receives the next command from the same initiator and
reports CHECK CONDITION status. The deferred error sense data is sent to
the initiator in REQUEST SENSE command.

A Unit Attention is used to report unusual events that occurred but weren't
related to any command sent by that initiator. An alternative to this is
the AEN (Asynchronous Event Notification) mechanism, but this really isn't
supported by most iniitators or targets. Examples of things reported with
Unit Attention:
(a) Reset reporting. A reset may have been caused by another initiator or
internally by the target, so it is not related to any commands that may
have been sent by a particular initiator that is recieving the sense data.
(b) Mode parameters changed by another initiator. This is a warning that
the target has been commanded to change at least one mode parameter by
another initiator and the change could affect the target's behavior with
this initiator also. Note that this type of unit attention is only set for
initiators other than the one that issued the MODE SELECT command.
(c) Microcode changed. This is a warning that new microcode has been loaded
onto the target and the target's benavior might be different.

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