void disableWriter() {
      this.writerEnabled = false;
    }
Free the space if the used size reaches acceptableSize() or one size block couldn't be
allocated. When freeing the space, we use the LRU algorithm and ensure there must be some
blocks evicted
@param why Why we are being called
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketCache.java