Path selection using a preferred path

When you specify a preferred path in the IODEVICE statement, the CSS always tries the preferred path first. If it is busy or unavailable, the CSS chooses the channel path in the control unit header rotation order that has the greatest likelihood of availability.

Channel paths to a device can belong to different logical partitions. I/O requests from any one logical partition use only the paths you assign to that logical partition. The rotation order for a device in a logical partition then follows the normal rotation order for the channel paths that belong to that logical partition.

When you specify a preferred path in an IODEVICE statement that is available to more than one logical partition, the device has a preferred path in all of its logical partitions in that CSS even though you can specify only one channel path per CSS in an IODEVICE statement as a preferred path. The preferred path for each LP is the first path in the rotation order that is defined to the LP beginning with the specified preferred path.

For example, assume that Control Unit Header 2 in Figure 1 has a rotation order of 01, 02, 07, 04 and paths 01 and 02 are in logical partition LP1 and paths 04 and 07 are in logical partition LP2. If device B specifies 02 as its preferred path, then 02 is the preferred path for logical partition LP1, and 07 is the preferred path for logical partition LP2.