code: private void preallocate() throws IOException {
      long position = fc.position();
      long triggerSize = Math.max(FSEditLog.preallocateSize / 100, 4096);
      if (position + triggerSize >= fc.size()) {
        if(FSNamesystem.LOG.isDebugEnabled()) {
          FSNamesystem.LOG.debug("Preallocating Edit log, current size "
              + fc.size());
        }
        fill.position(0);
        int written = fc.write(fill, position);
        if(FSNamesystem.LOG.isDebugEnabled()) {
          FSNamesystem.LOG.debug("Edit log size is now " + fc.size() +
              " written " + written + " bytes " + " at offset " + position);
        }
      }
    }
docs: allocate a big chunk of data