IBM DS6000 Series manual Hot spot avoidance, Preferred paths

8.6.7 Hot spot avoidance

Workload activity concentrated on a limited number of RAID ranks will saturate the RAID ranks. This may result in poor response times, so balancing I/O activity across any disk system is important. Spreading your I/O activity evenly across the available DS6000s will enable you to optimally exploit the DS6000 resources, thus providing better performance. I/O activity attributes to consider are to spread I/O activity across:

￿The two servers of the DS6000

￿The two loops of the DS6000

￿All available RAID ranks

￿Multiple DS6000s

8.6.8Preferred paths

In the DS6000, host ports have a fixed assignment to a server (or controller card). In other words, preferred paths to a server avoid having to cross the PCI-X connection. Therefore, there is a performance penalty if data from a logical volume managed by one server is accessed from a port that is located on the other server. The request for the logical volume 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, this 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.

When assigning host ports, always consider preferred pathing because the use of non-preferred paths will have a performance impact on your DS6000.

150DS6000 Series: Concepts and Architecture