An established best practice is to balance disk enclosures across back end loops in an HP StorageWorks 6400/8400 Enterprise Virtual Array (HP EVA6400/8400) product to ensure optimum performance and availability. 

To ensure optimum performance and availability, the HP EVA6400 requires that disk enclosures be balanced across the two available back end loops. For example, with six disk enclosures available, there should be three disk enclosures connected per loop. With 12 disk enclosures available, there should be six disk enclosures connected per loop, and so on.

To ensure optimum performance and availability, the HP EVA8400 requires that disk enclosures be balanced across the three available back end loops. For example, with six disk enclosures available, there should be two disk enclosures connected per loop. With 12 disk enclosures available, there should be four disk enclosures connected per loop, and so on.

Up to 18 disk enclosures are supported with the HP EVA6400 products and up to 27 disk enclosures are supported with the HP EVA8400 product.

 NOTE: There is a defined maximum of nine disk enclosures per loop.

As a best practice, attempt to keep back end loops as balanced as possible at all times. When the number of disk enclosures is not divisible by two for the HP EVA6400 or by three for the HP EVA8400, ensure that device enclosures do not differ by more than one disk enclosure per loop. It does not matter which loop has more or fewer disk enclosures. For example, with seven disk enclosures connected to an HP EVA6400, three disk enclosures should be connected to one loop and four disk enclosures to the second loop. With ten disk enclosures connected to an HP EVA8400, three disk enclosures should be connected to two of the loops and four disk enclosures to the third loop.

If your array has unbalanced loops (as described above) an offline maintenance period is needed to reconfigure the loops.