code: private void record(byte[] b, int off, int len) throws IOException {
          if(this.shouldDigest) {
              assert this.digest != null: "Digest is null.";
              this.digest.update(b, off, len);
          }
          tailRecord(b, off, len);
      }
docs: Record the given byte-array range for recovery later
  
  @param b Buffer to record.
  @param off Offset into buffer at which to start recording.
  @param len Length of buffer to record.
  
  @exception IOException Failed write to backing file.