title: how to merge 2 big files
body: <p>Suppose I have 2 files with size of 100G each. And I want to merge them into one, and then delete them. In linux
  we can use  </p>
  
  <p>cat file1 file2 > final_file  </p>
  
  <p>But that needs to read 2 big files, and then write a bigger file.  Is it possible just append one file to the other, so that no IO is required?  Since metadata of file contains the location of the file, and the length, I am wondering whether it is possible to change the metadata of the file to do the merge, so no IO will happen.</p>