IBM Midrange System DS4000 Series Hardware Manual page 265

If a logical drive modification is in progress, a status bar appears at the bottom of the window,
as shown in Figure 4-111.
Figure 4-111 Modification progress
Controller ownership
Each logical drive has a preferred controller of ownership. This controller normally handles all
I/O requests for this particular logical drive. In other words, each logical drive is owned by one
and only one controller. The alternate controller only takes over and handles the I/O requests
in case of a failure along the I/O path, for example, a defective host bus adapter or switch.
When defining logical drives, the system normally alternates ownership between the two
controllers as they are defined.
Situations can occur when all heavily stressed logical drives can reside on only one controller
and the other one handles only a small amount of all I/O requests. To balance the workload
between the controllers, the preferred ownership of a logical drive can be changed to the
other controller.
Important: Be sure that the operating system using the logical drive uses a multipath I/O
driver; otherwise, it loses access to the logical drive.
Balancing traffic is unfortunately not always a trivial task. For example, if an application
requires large disk space to be located and accessed in one chunk, it becomes harder to
balance traffic by spreading the smaller volumes among controllers.
Chapter 4. IBM System Storage DS planning and configuration
247