HP StorageWorks HSG60 and HSG80 Array Controller and Array Controller Software Troubleshooting Guide (EK-G80TS-SA. C01, March 2005)

Alternative Controller Operations
272 HSG60 and HSG80 Array Controller and Array Controller Software Troubleshooting Guide
Setting SCSI Fairness
Architecturally, the SCSI bus is inherently unfair in the way it drives bus priority.
For subsystems with six shelves, the bus priorities (highest to lowest) are set to 5,
4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, and 8. For subsystems with three shelves that
employ a split shelf configuration, where each shelf has a dual I/O module, the
bus priorities are set to 5, 4, 3, 2, 1, 0, and 8. As initiators for the disk device
buses, HSG60 and HSG80 array controllers are assigned a priority of 7 and 6. The
remaining SCSI IDs are for disk devices located in the shelves.
On extremely busy subsystems, performance on devices at ID 8 through 15 (split
shelves, 1, 0, 8) can degrade significantly if the higher IDs are extremely busy. If
SCSI disk devices are SCSI-3 devices, they adhere to an HP specification that
dictates that they have a fairness algorithm that levels the device performance
across a single SCSI bus. Under test conditions, where an equal I/O load is applied
to all targets on a SCSI bus, in an unfair condition, the top four SCSI target IDs
can consume up to 95 percent of the MBs transferred on the SCSI bus. Tests have
further noted that if this scenario occurs, latency for I/O completion to the host
can exceed 20 seconds.
This SET
controller SCSI_FAIRNESS=ON CLI command allows the
controller to identify all SCSI-3 disk devices and enable fairness algorithms.
Note: If you have already optimized your storage unit configuration to specific SCSI
IDs and buses, you should
not consider enabling this SCSI fairness with the SET
controller SCSI_FAIRNESS=ON command.
Note: HP StorageWorks HSG60 and HSG80 array subsystems were initially
designed to support only SCSI-2 devices. Currently, these subsystems support both
SCSI-2 and SCSI-3 devices. You should note that HP indiscriminately supplies either
SCSI-2 or SCSI-3 compliant devices as new or replacement spares for your subsystem.
Issuance of a specific device type is not guaranteed.