public long getSmallestReadPoint() {
    // We need to ensure that while we are calculating the smallestReadPoint
    // no new RegionScanners can grab a readPoint that we are unaware of.
    smallestReadPointCalcLock.lock(ReadPointCalculationLock.LockType.CALCULATION_LOCK);
    try {
      long minimumReadPoint = mvcc.getReadPoint();
      for (Long readPoint : this.scannerReadPoints.values()) {
        minimumReadPoint = Math.min(minimumReadPoint, readPoint);
      }
      return minimumReadPoint;
    } finally {
      smallestReadPointCalcLock.unlock(ReadPointCalculationLock.LockType.CALCULATION_LOCK);
    }
  }
@return The smallest mvcc readPoint across all the scanners in this region. Writes older than
        this readPoint, are included in every read operation.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegion.java