Pseudocode of Driver

We have implemented the Persistent Snapshot Driver Driver (PSDD) in Linux 2.2.5-15 as a separate module [7]. As discussed earlier, the pseudo layer will always exist, independent of whether the snapshot is on or off. The following pseudo code explains the request_fn of the pseudo device that implements snapshots.

psdd_request() {
 while (1) {
  get the request from the pseudo device queue;
  if request is snapshot block read {
     if mapping exists for this block
        read the block from the snapshot disk;
     else
        read the block from the original disk;
     continue;
  } 
  if request is original block read {
     read the block from the original disk;
     continue;      
  }
  if request is a write request {
     if mapping exists for this block
        write the block to the disk;
     else {
        read the block from  disk;
        copy to the snapshot disk;
        update the map;
        write the block to the disk;
     }
   }
 }
}