public long getNumEntriesOverflowOnDiskTestOnly() {
    long numEntriesOnDisk = 0;
    for (PartitionedRegion prQ : userRegionNameToShadowPRMap.values()) {
      DiskRegionStats diskStats = prQ.getDiskRegionStats();
      if (diskStats == null) {
        if (logger.isDebugEnabled()) {
          logger.debug(
              "{}: DiskRegionStats for shadow PR is null. Returning the numEntriesOverflowOnDisk as 0",
              this);
        }
        return 0;
      }
      if (logger.isDebugEnabled()) {
        logger.debug(
            "{}: DiskRegionStats for shadow PR is NOT null. Returning the numEntriesOverflowOnDisk obtained from DiskRegionStats",
            this);
      }
      numEntriesOnDisk += diskStats.getNumOverflowOnDisk();
    }
    return numEntriesOnDisk;
  }
Take will choose a random BucketRegionQueue which is primary and will take the head element
from it.
geode-core/src/main/java/org/apache/geode/internal/cache/wan/parallel/ParallelGatewaySenderQueue.java