RE: iSCSI:flow control, acknowledgement, and a deterministic recovery

["Douglas Otis" <dotis@sanlight.net>]

Santosh,

I see a few problems with this approach.  Tasks as defined in iSCSI do not
maintain connection allegiance.  The driver binds all SCSI commands to their
connection for the most resent association.  Although there are several
places within the iSCSI proposal that make reference to a task having a
connection allegiance, this is in error.  Commands and not tasks carry such
allegiance.  Your recovery scheme will not allow a satisfactory recovery
with a sequential device.  In this case, repeating the command is not a
solution.  As a result, one connection falter and it will become a difficult
situation.  In addition, you have no clue from iSCSI your delivery status.
You do not know if you are waiting for the target or if you are waiting for
the connection.  Some sequential devices have rather long time-outs with
these complications of deducing status created by the multiple connections.

The application will not know about these connection allegiance problems.
The iSCSI layer does not define interaction to provide additional
application status to allow these applications to respond in a manner that
may aid this situation nor should such additional information be required.
With your scheme the SCSI driver must examine the content of these commands
to make a guess as to the connection allegiance assignments.  Now the driver
is expected to understand what the intended action is of this SCSI
management command.  What signal is used to indicate a need for the iSCSI
immediate treatment?  The only obvious seems to be the task attribute
argument.  With the way iSCSI has defined iSCSI immediate, I would expect
those commands to be treated in a LIFO rather than the normal FIFO fashion.

Doug


> Douglas Otis wrote:
> >
> > With multiple connections, if you are not going to use a valid
> > CmdSN, or in your case a null CmdSN for all commands, then there
> > would be a need to include a timestamp to meet a timely delivery
> > requirement in the same manner as used in FC encapsulation.  IP
> > can deliver over any time period.  A command could arrive at any
> > time with respect to other connections.  With all of your feedback
> > now from just the SCSI layer, the SCSI layer is likely to have timed
> > out and restarted and now stray commands finally make an appearance
> > (the technician re-inserted the cable).  What did that do?  Yes,
> > if this were on a single connection, then TCP could provide some
> > assurances, (ignoring digests errors) but you must not make that
> > assumption nor can you assume all disruptions are symmetric.
>
> Doug,
>
> The below snippet from my last mail answered your above concern. The
> Abort Task is sent on the same connection as the command. (connection
> allegiance applied to the abort task as well). The Abort task pushes the
> stale data PDUs. There is no need for a timestamp on iSCSI PDUs.
>
> > > As for your second concern regarding I/O timeouts, there is
> no need for
> > > any timestamp. An I/O timeout is dealt with by an Abort Task.
> The abort
> > > task response guarantees that the abort reached the target and pushed
> > > all intermediate stale frames. Failure to complete Abort Task leads to
> > > higher level error recovery (ex : Logout, or some higher form of task
> > > mgmt).
>
> - Santosh