cp command and file changed

hi, I'd want to archive the whole directory. I'm thinking of usging cp -a, and not sure what it does if while in the process of copying, some files got changed or deleted. Will it
  case 1: stop?
  case 2:
         a: if file changed: ignore the changed and copy the old file and keep copying the rest?
          b: if file deleted: skip the file and keep copying the rest?
    or esle?