Obsoleting QAS in SPI-4

Mike Kosco Mike at MVBuilders.com
Tue May 2 16:14:53 PDT 2000


* From the T10 Reflector (t10 at t10.org), posted by:
* Mike Kosco <Mike at MVBuilders.com>
*
If we obsolete QAS in SPI-4, SPI-4 devices will suffer the same fairness
fate as the pre-SPI-3 devices.  All SPI-1, SPI-2 and SPI-4 devices will
have to live on the "dirty" end of the fairness stick.  I don't believe
there is a rule that says all devices on the bus must be QAS in order to
enable it.

I do agree that obsoleting QAS simplifies design and testing.  Has anyone
shipped devices with QAS support?

Regards,
Mike

Robert Elliott wrote:

> * From the T10 Reflector (t10 at t10.org), posted by:
> * relliott at hobbit.eng.hou.compaq.com (Robert Elliott)
> *
> As discussed in yesterday's Parallel SCSI working group meeting,
> here is the text for 00-228r0.  It should be on the web site
> shortly.
>
> I propose we make the QAS (quick arbitration and selection) protocol
> obsolete in SPI-4.
>
> Reasons:
> * Performance analysis of the fast-160 training patterns has shown
> that adding 2 microseconds is inconsequential.  QAS only promised
> to reduce arbitration time by a comparable amount.  If adding 2
> microseconds doesn't matter, subtracting 2 microseconds doesn't matter.
>
> * It will simplify logic design and testing if QAS is not included.
>
> * There is no fairness algorithm between QAS and non-QAS devices,
> so QAS should only be enabled when all devices on the bus support QAS.
> QAS can only be enabled when packetized protocol is enabled.
> Enclosure management devices and tape drives are unlikely to support
> QAS and packetized protocol any time soon.  Presence of these devices
> prohibits using QAS in many environments.
>
> * QAS depends on snooping traffic from other initiator-target
> connections.  It isn't clear whether this will work through
> all expanders.  There is no domain validation technique defined to
> verify that a domain supports QAS before enabling it.
>
> * The SCSI Trade Association (STA) has defined "Ultra 320" as
> including packetized protocol but not including QAS.
>
> --
> Rob Elliott
> Compaq Storage Products Division
> mailto:Robert.Elliott at compaq.com
>
> *
> * For T10 Reflector information, send a message with
> * 'info t10' (no quotes) in the message body to majordomo at t10.org

--
Mike Kosco
ChipCraft Corp.
(949)633-8383 Voice
(949)492-0082 Fax
www.MVBuilders.com


*
* For T10 Reflector information, send a message with
* 'info t10' (no quotes) in the message body to majordomo at t10.org




More information about the T10 mailing list