Obsoleting QAS in SPI-4
relliott at hobbit.eng.hou.compaq.com
Fri Apr 28 11:35:24 PDT 2000
* 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.
Compaq Storage Products Division
mailto:Robert.Elliott at compaq.com
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