IBM DS6000 Series manual Preferred pathing, Devserv Paths command showing preferred pathing

sections: Extent Pool Statistics and Rank Statistics. These statistics are generated from SMF record 74 subtype 8:

￿The ESS Extent Pool Statistics section provides capacity and performance information about allocated disk space. For each extent pool, it shows the real capacity and the number of real extents.

￿The ESS Rank Statistics section provides measurements about read and write operations in each rank of an extent pool. It also shows the number of arrays and the array width of all ranks. These values show the current configuration. The wider the rank, the more performance capability it has. By changing these values in your configuration, you can influence the throughput of your work.

Also, new response and transfer statistics are available with the Postprocessor Cache Activity report generated from SMF record 74 subtype 5. These statistics are provided at the subsystem level in the Cache Subsystem Activity report and at the volume level in the Cache Device Activity report. In detail, RMF provides the average response time and byte transfer rate per read and write requests. These statistics are shown for the I/O activity (called host adapter activity) and transfer activity from hard disk to cache and vice-versa (called disk activity).

12.2.9 Preferred pathing

In the DS6000, host ports have a fixed assignment to a server (or controller card). The DS6000 will notify the host operating system, in this case DFSMS (device support), if a path is preferred or not. Device support will then identify preferred paths to the IOS. I/Os will be directed to preferred paths to avoid crossing the PCI-X connection. The only time this will not be honored is when there are no preferred paths available. The software will then switch over to use non-preferred paths. There will be a slight performance penalty if the I/O is not executed over the preferred path. The I/O request and the data would have to be transferred across the bridge interface that connects both servers. These transfers add some latency to the response time. Furthermore, the bridge interface is also used to mirror the persistent memory and for other inter-server communication. It could become a bottleneck if too many normal I/O requests ran across it, although it is a high bandwidth, low latency, PCI-X connection. If the IOS support for preferred pathing is not implemented, sequential reads may drop by up to 50%. The response time in low stress environments may also increase by up to 10% to 20%.

New messages will inform the user when all preferred or the last preferred path is varied offline. Figure 12-2shows the output from the DEVSERV PATHS command. Now, this output displays preferred paths information.

DS P,9E02
IEE459I 09.14.03 DEVSERV PATHS 943
UNIT DTYPE	M CNT VOLSER	CHPID=PATH STATUS			DDC	ALT CU-TYPE
RTYPE	SSID CFW TC	DFW	PIN	DC-STATE CCA
9E02,33909	,O,000,339R53,38=+ 42=+
PATH ATTRIBUTES		PF	NP	SIMPLEX 02	02	2107
1750	9900 Y YY.	YY.	N
************************ SYMBOL DEFINITIONS ************************
O = ONLINE			+ = PATH AVAILABLE
PF = PREFERRED			NP = NON-PREFERRED

Figure 12-2 DEVSERV PATHS command showing preferred pathing

Figure 12-3,shows the output from the DISPLAY DEV command. The output has been enhanced to display the path attributes as preferred path (PF) or non-preferred path (NP).

248DS6000 Series: Concepts and Architecture