Re: iSCSI : EnableACA

[Santosh Rao <santoshr@cup.hp.com>]

Ralph Weber wrote:
> 
> Julian,
> 
> Regarding your discussion with Santosh...
> 
> > The task management functions for iSCSI contain a
> > description the behaviour I am talking about. This
> > was also subject to a long discussion on the list
> > that included the need for ACA behaviour for things
> > like task-set full, busy etc. - not considered for
> > ACA. For the later T10 is handling making ACA
> > behaviour available.
> >
> My question is... Exactly which what are the cases where
> T10 is NOT considering making ACA behavior available so
> that you believe the EnableACA function is necessary in
> iSCSI?
> 
> My belief is that every behavior that EnableACA covers
> but T10 does not needs to be taken to T10 to see if
> they can find a better way to extend ACA coverage to
> that area.

Julian,

My original proposal for this issue stated that ANY exception condition
on an I/O that had the NACA bit set in its control byte of the CDB must
result in ACA being established.

"Any exception condition" would include :
- I/O being aborted by the target due to a LU Reset, Target Reset or
Clear Task Set issued by another initiator.
- I/O being aborted by the initiator through the use of Abort Task.
- QUEUE FULL, BUSY, RESERVATION CONFLICT, etc status returned on the
I/O.

The caveat to this rule is that while ACA is already active, a second
one would not be established.

The point I'm trying to make is :

1) ACA is being used in this context to aid in the preservation of
strict command ordering. Any attempt to enforce strict command ordering
would require initiators to set the NACA bit in the cdb for their I/Os.
Such initiators would be ACA aware and Clear ACA capable.

The argument that initiators may not be able to perform "Clear ACA" and
so need an additional control [thru EnableACA] to prevent ACA from being
established is not applicable, because, such initiators would not set
NACA in their cdb's, and in that case, ACA would not be established.

2) ACA is a LU level construct and iSCSI is changing this granularity to
be a session level construct. For example, initiators could turn on ACA
to only 1 LU on which they had strong ordering requirements.

3) ACA is a ULP construct and any changes, if found necessary, should be
made in the ULP mode pages and not within iSCSI.

- Santosh
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