public void rollMetricsPeriod() {
    windowPeriods[windowIndex] =
      new Date((EnvironmentEdgeManager.currentTime() - (periodTimeInMinutes * 60 * 1000L)));
    hitCounts[windowIndex] = getHitCount() - lastHitCount;
    lastHitCount = getHitCount();
    hitCachingCounts[windowIndex] = getHitCachingCount() - lastHitCachingCount;
    lastHitCachingCount = getHitCachingCount();
    requestCounts[windowIndex] = getRequestCount() - lastRequestCount;
    lastRequestCount = getRequestCount();
    requestCachingCounts[windowIndex] = getRequestCachingCount() - lastRequestCachingCount;
    lastRequestCachingCount = getRequestCachingCount();
    windowIndex = (windowIndex + 1) % numPeriodsInWindow;
  }
Keep running age at eviction time
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/CacheStats.java