title: What is Mapped Buffer Pool / Direct Buffer Pool and how to increase their size?
body: <p><img src="https://i.stack.imgur.com/4Y9ga.png" alt="enter image description here"></p>
  
  <p>The screenshot of VisualVM was taken when I ran an IO intensive JVM program (written in Scala), heap size was 4 GB and only 2 GB were in-use. The JVM program uses memory mapped file.</p>
  
  <p>What does "mapped buffer pool" and "direct buffer pool" mean?</p>
  
  <p>Those pools seem to be very full. Since the JVM program uses memory mapped file, will I see increased performance if the pools were larger? If so, how to increase their size?</p>
  
  <p>The size of all mapped files are about 1.1GB in size.</p>