long sumPhysicalStorageUsage(Map<Short, Map<Short, ContainerStorageStats>> accountStorageStatsMap) {
    return accountStorageStatsMap.values()
        .stream()
        .flatMap(containerMap -> containerMap.values().stream().map(ContainerStorageStats::getPhysicalStorageUsage))
        .mapToLong(l -> l)
        .sum();
  }
Return the sum of physical storage usage in map.
@param accountStorageStatsMap The map from account id to container id to storage stats.
@return The sum of physical storage usage.
ambry-clustermap/src/main/java/com/github/ambry/clustermap/MySqlClusterAggregator.java