title: How to make loadable file system over the read only rootfs?
body: <p>I am working on embedded linux. I am trying to protect my rootfs by making it read only and mount fs file from the sdcard over the root.</p>
  
  <p>I need both fs to be merged.
  Any writes can be redirected to the fs file onto the sdcard.
  Reading from the read only rootfs still possible.</p>
  
  <p>I tried the following:</p>
  
  <p>$ cd /media/sdcard</p>
  
  <p>$ mount userfs /</p>
  
  <p>$ cd /</p>
  
  <p>$ echo a > a.txt</p>
  
  <p>But I receive error: 
  -sh: a.txt: Read-only file system</p>
  
  <p>Can any one help me to implement the needed functionality ?</p>