Compaq AA-RHGWC-TE - Manual (Page 68)

3.6.1.2.2 DS-DWZZH-05 Fair Arbitration
Although each cluster member system and storage controller connected to an
UltraSCSI hub are on separate SCSI bus segments, they all share a common
SCSI bus and its bandwidth. As the number of systems accessing the storage
controllers increases, it is likely that the adapter with the highest priority
SCSI ID will obtain a higher proportion of the UltraSCSI bandwidth.
The DS-DWZZH-05 UltraSCSI hub provides a fair arbitration feature that
overrides the traditional SCSI bus priority. Fair arbitration applies only to
the member systems, not to the storage controllers (which are assigned
higher priority than the member system host adapters).
You enable fair arbitration by placing the switch on the front of the
DS-DWZZH-05 UltraSCSI hub to the Fair position (see Figure 3–4).
Fair arbitration works as follows. The DS-DWZZH-05 UltraSCSI hub is
assigned the highest SCSI ID, which is 7. During the SCSI arbitration phase,
the hub, because it has the highest priority, captures the SCSI ID of all host
adapters arbitrating for the bus. The hub compares the SCSI IDs of the host
adapters requesting use of the SCSI bus, and then allows the device with the
highest priority SCSI ID to take control of the SCSI bus. That SCSI ID is
removed from the group of captured SCSI IDs prior to the next comparison.
After the host adapter has been serviced, if there are still SCSI IDs retained
from the previous arbitration cycle, the next highest SCSI ID is serviced.
When all devices in the group have been serviced, the DS-DWZZH-05
repeats the sequence at the next arbitration cycle.
Fair arbitration is disabled by placing the switch on the front of the
DS-DWZZH-05 UltraSCSI hub in the Disable position (see Figure 3–4).
With fair arbitration disabled, the SCSI requests are serviced in the
conventional manner; the highest SCSI ID asserted during the arbitration
cycle obtains use of the SCSI bus.
______________________ Note _______________________
Host port SCSI ID assignments are not linked to the physical port
when fair arbitration is disabled.
The DS-DWZZH-05 reserves SCSI ID 7 regardless of whether fair
arbitration is enabled or not.
3–12 Shared SCSI Bus Requirements and Configurations Using UltraSCSI Hardware