Comment 9 for bug 361733

Here is an implementation of load balancing; this chooses the closest drive for a read UI, which results in doing sequential IO from one drive (and this works a lot better than round-robin per-IO: dm mapping happens before IO request merging - doing round-robin results in no request merging and terrible performance).

Random IO (e.g. kernel builds, regular use) shows up on both disks in my mirror set, often in similar amounts.

Future iterations can build on this to do more advanced algorithms.