Unit attention on FCP devices
rsnively at Brocade.COM
Tue Aug 29 09:33:10 PDT 2000
* From the T10 Reflector (t10 at t10.org), posted by:
* Robert Snively <rsnively at Brocade.COM>
FCP devices work the way you expect them to. The Unit Attention is
cleared with the presentation of the status and the sense information.
The stacking up of commands against a device that has established
a Unit Attention condition will create similar behaviors in
serial and parallel port cases. Basically, when Unit Attention is
presented, the driver must sort out those operations which may have
been unsuccessful from those that are successful. In parallel
SCSI, the Unit Attention may even be cleared by commands that do
not obtain sense information.
> -----Original Message-----
> From: Edward A. Gardner [mailto:eag at ophidian.com]
> Sent: Monday, August 28, 2000 8:28 AM
> To: T10 Reflector
> Subject: Unit attention on FCP devices
> * From the T10 Reflector (t10 at t10.org), posted by:
> * "Edward A. Gardner" <eag at ophidian.com>
> I recently encountered a question about the operation of
> unit attention on
> FCP devices. While I think I know how it has to work, I
> cannot find a clear
> statement of this anywhere (SAM, SAM-2, FCP, FCP-2).
> The specific question is what event clears a unit attention
> condition on an
> FCP device (or more generally on any device that supports autosense).
> Consider the following events:
> 1. An initiator is accessing a LUN on an FCP target.
> 2. A unit attention condition occurs on the LUN during some
> period when no
> commands or tasks are outstanding to it. Perhaps someone
> changes the media
> on a removable media device. The event specifically does
> not disturb the
> connection (process login) between the initiator and target.
> 3. The initiator sends a command (NACA=0) to the target.
> This command
> encounters the unit attention condition, the target returns
> CHECK CONDITION
> and autosense data describing the unit attention condition.
> Note that the
> CA condition that occurs by returning CHECK CONDITION is
> immediately cleared
> by the simultaneous return of autosense data (I would like
> to say that no CA
> condition occurs, but that isn't SAM terminology).
> At this time, has the return of autosense data describing
> the unit attention
> condition cleared the unit attention, or does it still exist
> until the
> initiator issues an explicit REQUEST SENSE to clear it?
> On the one hand I believe it is necessary that the unit
> attention condition
> still exist until an explicit REQUEST SENSE. Otherwise
> commands that are in
> flight, issued before the initiator was aware of the unit attention
> condition, may be executed against the wrong media (or
> against whatever
> change the unit attention was reporting).
> On the other hand the words in SAM and SAM-2 appear to
> suggest that the unit
> attention condition will be cleared by the return of
> autosense data. SAM
> and SAM-2 use the phrase "if a request for sense data is
> received" (clause
> 5.6.5). One can interpret autosense as a request for sense
> data. Clause
> 18.104.22.168 states "The return of sense data in this way [autosense] is
> equivalent to an explicit command from the application
> client requesting
> sense data". That is, returning autosense data is
> equivalent to a REQUEST
> SENSE, and REQUEST SENSE clears unit attention.
> The one thing I have no doubt about is that the current
> wording describing
> unit attention handling is ambiguous and confusing; I intend
> to generate
> some comments to clarify it. But first I want to make
> certain whether FCP
> devices work the way I think they do.
> Edward A. Gardner eag at ophidian.com
> Ophidian Designs 719 593-8866 voice
> 1262 Hofstead Terrace 719 593-8989 fax
> Colorado Springs, CO 80907 719 210-7200 cell
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