Currently our MySQL cluster is running on 6 data nodes in a mirrored setup. There data nodes make up the dataset, and then those are mirrored to a second set of 3 nodes.

This is a little bit different than most people would recommend. Officially MySQL wants you to grow you cluster in 2's. IE: 2, 4, 8, 16 etc. But as of MySQL 5.1.15 you no longer have to follow that pattern. You can now use an "odd" number of data nodes. With testing and everything, we have been running the cluster for about 3 months without any problem.