Obsoleting QAS in SPI-4
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
I do agree that obsoleting QAS simplifies design and testing. Has anyone
shipped devices with QAS support?
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
> I propose we make the QAS (quick arbitration and selection) protocol
> obsolete in SPI-4.
> * 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
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