One one small LEMP VPS I manage, I noticed munin graphs that showed anywhere between 5-50 MB/second of disk IO. Since the VM has an SSD instead of traditional spinning hard drive, performance wasn't too bad, but all that disk I/O definitely slowed things down.
I wanted to figure out what was the source of all the disk I/O, so I used the following techniques to narrow down the culprit (spoilers: it was MySQL, which was using some swap space because it was tuned to use a little too much memory).
First up was
iotop, a handy top-like utility for monitoring disk IO in real-time. Install it via yum or apt, then run it with the command
sudo iotop -ao to see an aggregated summary of disk IO over the course of the utility's run. I let it sit for a few minutes, then checked back in to find: