Moodle database server is the bottleneck

We try to do stress test on Moodle 2.3.1 with 4 web servers and 1 MySQL database server. However, we find that there is a bottleneck in the Moodle MySQL database server. The write IO has reached the peak, thus, we have many long running queries when Moodle acquires a lock on mdl_session. However, running Moodle with 3 web servers has better performance than 4 web servers. May i ask how we can improve the write IO? We are already using Paravirtual SCSI controller on a SAN. Besides, do you think using MySQL clusters with Moodle is a good solution?