RE: iscsi: rev 05 2.5.1 requires ACA support

[julian_satran@il.ibm.com]

David,

As SCSI can liberally set or reset the bits in CDB and we don't want iSCSI
to have to look into it (we managed to avoid this until now) I though that
it will serve us well to request ACA support so that we get it at least
from all the new drivers.

As with many of other tiny inconsistencies we are bound to encounter this
stems in part from the fact that SAM considers the specific protocol (FCP,
SBP etc.) as a blending of transport and SCSI (see the many protocol
specific parameters and behaviors) without specifically addressing the
"separability" issue that the layering purists among us demand.

Regards,
Julo


Black_David@emc.com on 16/03/2001 16:21:53

Please respond to Black_David@emc.com

To:   Julian Satran/Haifa/IBM@IBMIL, ips@ece.cmu.edu
cc:
Subject:  RE: iscsi: rev 05 2.5.1 requires ACA support




If that's the case, then the wording that Ralph
pointed out needs to be modified to indicate that
ACA is used only when appropriate.

--David

> -----Original Message-----
> From:   julian_satran@il.ibm.com [SMTP:julian_satran@il.ibm.com]
> Sent:   Friday, March 16, 2001 2:34 AM
> To:     ips@ece.cmu.edu
> Subject:     RE: iscsi: rev 05 2.5.1 requires ACA support
>
>
>
> We are aware of the support for ACA missing from some drivers.
> The situation is even exacerbated by the fact that certain exceptions
that
> are not errors per-se
> will require ACA to be fired to accomodate for commands in flight (like
> reservations, busy, task-set-full).
>
> However actions at the initiator can be fine-tuned with the NACA bit in
> CDB
> and the ACA atrribute for the task.
>
> Julo
>
> Black_David@emc.com on 16/03/2001 03:46:29
>
> Please respond to Black_David@emc.com
>
> To:   Julian Satran/Haifa/IBM@IBMIL, ips@ece.cmu.edu
> cc:
> Subject:  RE: iscsi: rev 05 2.5.1 requires ACA support
>
>
>
>
> > After an error if you have commands in flight you want them all dropped
> > until you specifically reset the ACA and restart the queue (prevent
> things
> > to be executed out of order).
>
> The T10 folks will have to go after this one in more detail, but Julian's
> above statement is correct *only* if the commands in flight depend on
> the one that caused the error (i.e. executing them out of order will
cause
> problems).  This is generally not the case for disks where the usual
> practice is to enforce command execution order dependencies
> (e.g., database write ordering) in the operating system and applications
> by waiting for responses (yes it's possible to do better, but lots of
> existing software doesn't).  The result is that commands in
> flight can be executed in arbitrary order with arbitrary ones of them
> failing without causing further difficulties.  As Ralph has pointed out,
> most hosts do not use ACA for disk-based storage, and if iSCSI
> always does ACA, this will cause nasty integration issues.
>
> Just to stir the pot further, I believe Fibre Channel provides a negative
> example, because if FC drops a frame (which is not a good idea,
> but can still happen), the FC component that dropped the frame has no
> clue about what ACA is, or how to get the target (which is not the
> same piece of hardware) to enter ACA state.  Both Class 2 and Class 3
> are vulnerable to this.
>
> Tapes are another matter - do we still have a tape expert on the list?
>
> I thought where we were headed on ACA was something along the
> lines of:
> - Targets MUST implement.
> - Initiators MAY use.
> - Initiator support for ACA is NOT REQUIRED.
> which would imply a text key for Initiators to tell Targets
> whether ACA behavior is expected.  Did I miss something?
>
> --David
> ---------------------------------------------------
> David L. Black, Senior Technologist
> EMC Corporation, 42 South St., Hopkinton, MA  01748
> +1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
> black_david@emc.com       Mobile: +1 (978) 394-7754
> ---------------------------------------------------
>
>
>