Close
All I know is that for the few days prior to the RAID scrub, the server was having no lag even during peak and was extremely responsive. Your performance graphs show the same anomaly, and also that this period of excellent stable performance started a half a week after the upgrade to 12.04.

http://img841.imageshack.us/img841/3562/diskstatslatencyweek.png
http://img850.imageshack.us/img850/558/diskstatsutilizationwee.png
http://img440.imageshack.us/img440/2008/iostatiosweek.png