title: Creating unflushed file output buffers
body: <p>I am trying to clear up an issue that occurs with unflushed file I/O buffers in a couple of programs, in different languages, running on Linux.  The solution of flushing buffers is easy enough, but this issue of unflushed buffers happens quite randomly.  Rather than seek help on what may cause it, I am interested in how to create (reproduce) and diagnose this kind of situation.</p>
  
  <p>This leads to a two-part question:</p>
  
  <ol>
  <li><p>Is it feasible to artificially and easily construct instances where, for a given period of time, one can have output buffers that <em>are known</em> to be unflushed?  My searches are turning up empty.  A trivial baseline is to hammer the hard drive (e.g. swapping) in one process while trying to write a large amount of data from another process.  While this "works", it makes the system practically unusable: I can't poke around and see what's going on.</p></li>
  <li><p>Are there commands from within Linux that can identify that a given process has unflushed file output buffers?  Is this something that can be run at the command line, or is it necessary to query the kernel directly?  I have been looking at <code>fsync</code>, <code>sync</code>, <code>ioctl</code>, <code>flush</code>, <code>bdflush</code>, and others.  However, lacking a method for creating unflushed buffers, it's not clear what these may reveal.</p></li>
  </ol>
  
  <p>In order to reproduce for others, an example for #1 in C would be excellent, but the question is truly language agnostic - just knowing an approach to create this situation would help in the other languages I'm working in.</p>
  
  <hr>
  
  <p>Update 1: My apologies for any confusion.  As several people have pointed out, buffers can be in the kernel space or the user space.  This helped pinpoint the problems: we're creating big dirty kernel buffers.  This distinction and the answers completely resolve #1: it now seems clear how to re-create unflushed buffers in either user space or kernel space.  Identifying which process ID has dirty kernel buffers is not yet clear, though.</p>