The Rebalance script for IBM SVC has been updated. This is the first update I have seen since 2010. This release of the SVCTools package will now work on the Storwize family of products without modification and it can rebalance Easy Tier managed disk groups.
Why use this? Normally in SVC (and always with Storwize products), each MDisk is a different RAID array. In those cases, when you add MDisks (arrays) to a pool (MDiskGrp) then you are adding extra spindles to that pool. By rebalancing extents from existing volumes onto new MDisks, existing volumes will almost always get a performance boost. It also means that you free up space on the older MDisks so that when you later create new volumes in that pool, they will get a chance to use extents across a wider range of MDisks (old and new, not just new). This is particularly useful if the pool was full when you added extra space.
Of course if you are using Easy Tier, then the hardest working extents will already be on SSD, so rebalancing may not be as beneficial. Note that Easy Tier will be disabled while the balance.pl script is running and extents on SSD (presumably put there by Easy Tier) will not be rebalanced or moved. Also if your backend disk is already wide-striped (like the IBM XIV) then rebalancing is not necessary.