@Override
  public String toString() {
    return "ioengine=" + this.getClass().getSimpleName() + ", capacity="
      + String.format("%,d", this.capacity);
  }
Construct the ByteBufferIOEngine with the given capacity
@throws IOException ideally here no exception to be thrown from the allocator
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/ByteBufferIOEngine.java