I have just finished debugging an iSCSI storage system where the I/O on the disk appeared to be significantly higher than the I/O coming from the running programs. I discovered that the top of the iotop program can confirm this:
Total DISK READ : 63.46 M/s | Total DISK WRITE : 42.03 M/s
Actual DISK READ: 15.28 M/s | Actual DISK WRITE: 112.13 M/s