public long overflow() {
      return totalSize - bucketSize;
    }
Used to group bucket entries into priority buckets. There will be a BucketEntryGroup for each
priority (single, multi, memory). Once bucketed, the eviction algorithm takes the appropriate
number of elements out of each according to configuration parameters and their relative sizes.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketCache.java