How to find out the cause of EC2 EBS I/O requests?

I have a EC2 instant. The Account Activity is showed below. enter image description here

My EBS I/O requests is very large. I run socket.io with Node.js which has about 1,000 users connecting the socket. But the traffic of the socket is very small. About a data transport(a string) per 10 minutes. So, is there a way to find out the cause? Thanks.