IBM Midrange System DS4000 Series Hardware Manual page 215

Figure 4-61 Customize logical drive parameters
b. The segment size is chosen according to the usage pattern. For custom settings, you
can directly define the segment size.
c. Dynamic cache read prefetch is important for multimedia applications that use
sequential I/O, by allowing the controller, while it is reading and copying host-requested
data blocks from disk, to copy additional data blocks into the cache.
d. The preferred controller handles the logical drive normally if both controllers and I/O
paths are online. You can distribute your logical drives between both controllers to
provide better load balancing between them. The default is to alternate the logical
drives on the two controllers.
It is better to spread the logical drives by the load that they cause on the controller. If
you do not know the expected access pattern for each logical drive, you can evaluate it
by using the performance monitor option integrated with the Storage Manager client.
Based on data gathered from the Performance Monitor, move some logical drives to
the other preferred controller to balance the load if required (see 4.9.7, "Monitoring and
alerting" on page 223, and 4.1.2, "Logical drives and controller ownership" on
page 114 for more information).
Chapter 4. IBM System Storage DS planning and configuration
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