title: What is the definition of I/O, and what actions are classified as I/O?
body: <p>When I was studying about linux cache, I found that I've been using the term I/O without knowing its proper definition.</p>
  
  <p>I quote from someone who responded to me about Linux page cache, "Linux kernel populates (and uses) page cache when you are doing i/o, not when you open a file"</p>
  
  <p>Basically, I treated the term I/O as any sys command, but turns out opening a file is not considered I/O.</p>
  
  <p>Furthermore, the definition of I/O is rarely discussed or documented online.</p>
  
  <p>Which leads to my question. What is the definition of I/O (particularly in the context of accessing disks), and what actions can be classified as I/O?</p>