public long getReservedBytes() {
    return memoryBlock.getUsedMemoryInBytes();
  }
Cancel the specified memory reservation. If the reservation has finished, do nothing.

@param future The future returned from {@link #reserve(String, String, String, long, long)}
@return If the future has not complete, return the number of bytes being reserved. Otherwise,
    return 0.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/memory/MemoryPool.java