RE: iSCSI: Why is ACA optional?

[Charles Monia <cmonia@NishanSystems.com>]

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