Obsolete untagged tasks proposal

wrstuden at wasabisystems.com wrstuden at wasabisystems.com
Thu Aug 14 15:37:55 PDT 2003

* From the T10 Reflector (t10 at t10.org), posted by:
* wrstuden at wasabisystems.com
On Thu, 14 Aug 2003 dcuddihy at attotech.com wrote:

> Ok...  It makes sense that CA is no longer needed as long as transports do
> autosense.
> Still, why continue with ACA?  the iSCSI folks seem to have done a large
> amount of hand wringing WRT ACA, which, whether obsolete or not, is rarely
> used.

Ironically, for tape. :-)

The explaination I was given was for this scenario:

You have a number of commands to write a tape, all with ORDERED
attributes. They also have ACA set.

If the tape runs out partway through, that command will error out (with a
residual count), and all the other tasks will be stalled (Ok, that's not
the technical terms). You then send in tape changer/loader commands with
the ACA attribute. Next you send the un-written part with a write that has
the ACA attribute. Then you clear ACA, and the previous commands resume.

I can't say too much more, as I'm repeating someone else's explaination.
But the idea is that you can stop pending commands and sneak error-
recovery operations in front of them. Also, for the example above, if the
tape switch behavior were pre-selected, the recovery steps could happen in
a scsi driver; the tape application that queued the commands need not know
they had an issue.

Take care,

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