HP StorageWorks SAN Virtualization Services Platform Best Practices Guide (5697-0935, May 2011)

3 Array configuration
To ensure optimum performance and availability of the SVSP configuration, it is important to
understand how logical units (called back-end logical units or LUs) are:
• Presented by the array to SVSP
• Seen and accessed by SVSP
Back-end LU configuration
A back-end LU is a logical unit presented directly to the SVSP from a storage array. The question
arises as to how many LUs should be presented from the array to the DPMs. The answer is
determined by the number of array ports being presented to the DPMs.
At a minimum, the number of LUs you present to the DPM should equal the number of array ports
presented to the DPM. For example, if you are presenting four array ports to the DPM, then you
should present four back-end LUs at a minimum. If you are presenting eight ports from a storage
array to the DPMs, then you need to present, at a minimum, eight back-end LUs from this array to
the DPMs.
Be aware that I/O is transmitted from the DPM to a LUN along only one path. Using the minimum
number of back-end LUs however does not guarantee that the DPM will use all available paths
because the DPM does not have a load balancing algorithm. This may result in a path transmitting
I/O to more than one LU, creating a situation where I/O may become concentrated on some
paths, while other paths remain unused (except as a result of a path failure).
The only way to ensure that all paths from the DPM to the array are used, that is, each available
path transmits I/O to at least one back-end LU, is to present double the number of LUs to the DPM
as there are paths. For example, if 4 paths are presented from the array to the DPM, then present
8 back-end LUs to the DPM from this array. If 8 paths are presented from the array to the DPMs,
then present 16 back-end LUs to the DPM from this array. Doubling the number of back-end LUs
relative to the number of paths available will help ensure that the DPM will use each of the available
paths.
Figure 9 Match minimum virtual disk presentation to array ports
Carefully monitor all arrays for latency and saturation, not only to obtain the best performance,
but also to ensure that events are handled in a timely manner. An array that is heavily loaded may
compromise failover and possibly the stability of the SAN.
18 Array configuration