“Odd patterns of I/O latency can be hidden by line graphs and summary statistics, and revealed by histograms and heat maps. In my previous post I showed my Linux iosnoop tool, which can trace block device I/O along with timestamps and latency. This information can be visualized, revealing any odd patterns.”

Source: http://www.brendangregg.com/blog/2014-07-23/linux-iosnoop-latency-heat-maps.html