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Table 2–1: DS-DWZZH UltraSCSI Hub Maximum Configurations (cont.)
DS-DWZZH-03 DW-DWZZH-05 Disk Drives
a
Personality
Module
bc
1 1 2 Installed
0 1 3 Installed
a
DS-DWZZH UltraSCSI hubs and disk drives may coexist in a storage shelf. Installed disk drives are not
associated with the DS-DWZZH UltraSCSI hub SCSI bus segments; they are on the SCSI bus connected to
the personality module.
b
If the personality module is installed, you can install a maximum of four DS-DWZZH-03 UltraSCSI hubs.
c
The personality module must be installed to provide a path to any disks installed in the BA356 or
DS-BA356 shelf.
2.3 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 2–1).
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 2–1).
With fair arbitration disabled, the SCSI requests are serviced in the
Using the DS-DWZZH-05 UltraSCSI Hub in a Cluster Configuration 2–3