Large disk arrays can suffer from an imbalance of data across their RAID/parity groups. This is inevitable even if you plan your LUN allocation as data profiles change over time and storage is allocated and de-allocated.
So, tools are available. Think of EMC Optimizer, HDS Cruise Control and Volume Migrator.
I’ve put a poll up on the blog to see what people think – I have my own views and I’ll save them until after the vote closes next week.