RE: iSCSI: Why is ACA optional?

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

Charles,

For tape applications, not providing Sense Information within the iSCSI
response should remove much of the troubles caused by an ACA mandate.  In
otherwords, not seeing Sense within the Auto-Sense PDU should not be seen as
an error and thus all is well.

Doug

> Hi Julo:
>
> A blanket requirement for mandatory ACA support would make almost
> all legacy
> devices noncompliant.  I don't think the T10 community would be willing to
> support that any time soon.
>
> Charles
> > -----Original Message-----
> > From: julian_satran@il.ibm.com [mailto:julian_satran@il.ibm.com]
> > Sent: Wednesday, November 29, 2000 9:21 PM
> > To: ips@ece.cmu.edu
> > Subject: RE: iSCSI: Why is ACA optional?
> >
> >
> >
> >
> > Charles,
> >
> > That is probably the path we should be taking although I
> > wonder why would
> > T10 not mandate as it
> > as this thing affects all interconnects. We might then (as
> > with the name
> > mapping) see it happen in T10.
> >
> > Regards,
> > Julo
> >
> > Charles Monia <cmonia@NishanSystems.com> on 30/11/2000 00:06:29
> >
> > Please respond to Charles Monia <cmonia@NishanSystems.com>
> >
> > To:   "Ips (E-mail)" <ips@ece.cmu.edu>
> > cc:
> > Subject:  RE: iSCSI: Why is ACA optional?
> >
> >
> >
> >
> > Hi Julo:
> >
> >
> >
> > > -----Original Message-----
> > > From: julian_satran@il.ibm.com [mailto:julian_satran@il.ibm.com]
> > > Sent: Wednesday, November 29, 2000 11:18 AM
> > > To: ENDL_TX@computer.org
> > > Cc: ips@ece.cmu.edu
> > > Subject: Re: iSCSI: Why is ACA optional?
> > >
> > >
> > >
> > >
> > > Ralph,
> > >
> > > That is an interesting argument.
> > >
> > > However, now that disks have become more complex and can
> > > queue tasks how
> > > would you
> > > handle having one task rejected because the queue was full
> > > and the next
> > > (still in flight) accepted because
> > > some slot became available?
> > >
> >
> > The case you mention certainly addresses most but not all
> > such occurences.
> > There are other transient exceptions, such as commands that
> > terminate with
> > a
> > BUSY or ACA ACTIVE status, that have the same effect but do
> > not themselves
> > result in an ACA condition.
> >
> > > And this might even happen inside the host (possibly a large
> > > SMP) that with
> > > SMP would not have to care coordinating tasks but without ACA
> > > will have to
> > > strictly serialize access.
> > >
> > > IMHO TODAY not mandating ACA is a mistake (IBM 360 disk
> > > controllers had it
> > > 30 years ago for the very reason I quoted).
> > > I any case iSCSI can do little to ease the pain - except to
> > > point out to
> > > those that plan using disk subsystems without ACA to rely on status
> > > numbering and issue commands one by one.
> > >
> >
> > I believe iSCSI always has the option of making ACA support
> > mandatory (in
> > the same way that autosense support is mandatory).  If so,
> > for practical
> > reasons it will be up to the initiator's iSCSI stack to do so
> > in a way that
> > is transparent to the parts of the I/O driver stack above the
> > iSCSI layer.
> >
> > Charles
> >
> >
> >
>