During the time when the system is running, the SCSI bus is generally either active or
idle. If active, the bus is busy transmitting data from one device to another; if idle, it
is available for a device to begin sending a command or data. When a device decides it
wants to use the bus, it "bids" for control of the bus. It is also possible that
other devices on the bus will want to use it at the same time, so they too may
"bid" for control. A specific method is used to resolve these requests and
decide which device gets to use the bus first; this is based to some extent on the
devices' respective priority levels. This process is called arbitration.
While arbitration works fine in regular SCSI configurations, it introduces overhead.
During the time that arbitration is going on, no data is being transferred on the bus, so
it makes sense that doing this faster will allow improved performance of the entire SCSI
subsystem. To this end, the SPI-3 standard defined a
feature that reduces the overhead required for arbitration. This feature is called quick
arbitration and selection or QAS. You may also see it called by the name it
carried during development, quick arbitration and select; IBM calls it quick
arbitration select and Adaptec, simply quick arbitrate. These are all
different names for the same feature.
In a nutshell, QAS works by reducing the number of times arbitration must occur on the
bus. When the feature is used, a device waiting for the bus can grab it more quickly after
the last device on the bus sends the signal that it is done, without having to begin a new
arbitration process. Provision is made in the specification to ensure that one device does
not "dominate" the bus by "unfairly" blocking out other devices that
may be of a lower priority or may not implement QAS.
Quick arbitration and selection is one of the five "optional" features of Ultra3 SCSI. It was not included as one of the required
features for hardware meeting the Ultra160 specification,
but is present in Ultra160+ devices.
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