Disk I/O

I have a two tier structure, S1 Runs apache, Drupal,APC and memcached, S2 runs mysql.
Moderate traffic about 10K visits/day.
I recently started to use a new monitoring programme and it showing lot of IOwait happening on the server S1. Further analysis shows all that IOwait is due to apache. the disk IO monitor shows most of the IO is write. Which is expected as reads (although several times higher) are happening through memcache and network from mysql. I also depend on CDN to distribute most of the static files.

I have confirmed NO swapping is going on here. Also NO NFS here.

Background: The Drupal 6 site used to work very well on a VPS until a few months ago. However in order to reduce the server cost as well as to improve performance of the site even more, we decided to move to the cloud and Pantheon. We setup a pantheon server and moved a copy of the site on rackspacecloud and tested it using Jmeter with 1000 users and tested continuously for 4 hours and the load never went more than 1. The Jmeter report showed only .4% error(which was normal i think).