Question: Deferred Sense reporting priority

Joseph C. Nemeth jnemeth at
Mon May 4 10:27:34 PDT 1998

* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
* "Joseph C. Nemeth" <jnemeth at>
Like Terry, I'm in the process of implementing a high-level protocol, and I
have a question regarding Unit Attention in a fibre-channel fabric

My understanding of the Unit Attention condition is that it is posted on a
per-initiator basis. That is, if there are multiple initiators on a SCSI
bus, and a device powers up, each initiator should see the PON Unit
Attention condition (the first initiator to poll the device should not clear
Unit Attention for other initiators). In fact, some of the Unit Attention
conditions (Mode Page parameter changes, for example), are ONLY posted for
initiators other than the one which performed the Mode Select, which is
pretty useless if any initiator can clear the Unit Attention condition for
all the rest of the initiators.

In the SCSI-2 environment, implementation could be done simply: one way I've
done it is to simply have a 16-bit bit-mask for each separate Unit Attention
condition, one bit for each possible initiator on the (wide) SCSI bus. When
that initiator issues a command and the bit is set, the initiator receives a
Check Condition status, at which point I generate the appropriate sense data
and clear the bit.

In the fibre channel private loop environment, this kind of a strategy
scales reasonably well, since there can be only 126 addresses on the loop. A
128-bit bit-mask per Unit Attention condition is not unreasonable.

In a public loop environment, this strategy won't work at all, since the
number of potential initiator fabric addresses (through the FL port) is
astronomical. Plus, I'm puzzled how I'm supposed to manage this at all. As I
understand it, the FL port hides most of the initiator's full fabric address
|from me anyway -- to me, as a loop device, it looks like the FL port itself
is doing all of the initiating, and it keeps track of everything based on
the exchange identifier and magically makes my responses go to the right
place. So as a loop target, I won't necessarily know whether I'm getting two
commands from one initiator on the fabric, or two commands from two
different initiators.

Have I missed something?

-----Original Message-----
From: Gerry_Houlder at <Gerry_Houlder at>
To: t10 at Symbios.COM <t10 at Symbios.COM>
Date: Friday, April 24, 1998 4:42 PM
Subject: Re: Question: Deferred Sense reporting priority

>* From the T10 (formerly SCSI) Reflector (t10 at, posted by:
>* Gerry_Houlder at
>Most targets only retain the "latest" sense data rather than remember the
>last N events.

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