title: What to put in a binary data file's header
body: <p>I have a simulation that reads large binary data files that we create (10s to 100s of GB).  We use binary for speed reasons.  These files are system dependent, converted from text files on each system that we run, so I'm not concerned about portability.  The files currently are many instances of a POD struct, written with fwrite.</p>
  
  <p>I need to change the struct, so I want to add a header that has a file version number in it, which will be incremented anytime the struct changes.  Since I'm doing this, I want to add some other information as well.  I'm thinking of the size of the struct, byte order, and maybe the svn version number of the code that created the binary file.  Is there anything else that would be useful to add?</p>