public double getHitCachingRatio() {
    double requestCachingCount = getRequestCachingCount();

    if (requestCachingCount == 0) {
      return 0;
    }

    return getHitCachingCount() / requestCachingCount;
  }
Keep running age at eviction time
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CacheStats.java