public static long getZeroCopyBytesRead() {
    synchronized (readStatistics) {
      return readStatistics.totalZeroCopyBytesRead;
    }
  }
Because HDFS ReadStatistics is calculated per input stream, it is not feasible to update
the aggregated number in real time. Instead, the metrics are updated when an input stream
is closed.
hbase-server/src/main/java/org/apache/hadoop/hbase/io/FSDataInputStreamWrapper.java