RE: iSCSI: Why is ACA optional?

[Charles Monia <cmonia@NishanSystems.com>]

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
> 
> 
>