Setting innodb_buffer_pool_size in for Mysql
I think 80% is a little bit too much, I suggest 60%. Otherwise you'll risk swapping.

If you're data and indexes do not fit into ram (innodb_buffer_pool_size) you'll experience more disk reads. Which of course is slower than reading from RAM, especially when you have slow disks.

Monitoring disks (IOPS) can be done using iostat or pt-diskstats.