public long getOccupiedSize() {
   return (long) (numBlocks - backingStore.getBlocksRemaining()) * (long) blockSize;
 }
 @Override
 public long getBlockCount() {
   return numBlocks - backingStore.getBlocksRemaining();
 }
 public long getFreeSize() {
   return (long) backingStore.getBlocksRemaining() * (long) blockSize;
 }