public static Region<Object, Object> getSyncOverFlowOnlyRegion(Cache cache,
      DiskRegionProperties diskRegionProperties) {
    if (diskRegionProperties == null) {
      diskRegionProperties = new DiskRegionProperties();
    }
    diskRegionProperties.setPersistBackup(false);
    diskRegionProperties.setSynchronous(true);
    diskRegionProperties.setOverflow(true);
    return getRegion(cache, diskRegionProperties, Scope.LOCAL);
  }
A testing helper factory to get a disk region with the desired configuration
geode-junit/src/main/java/org/apache/geode/internal/cache/DiskRegionHelperFactory.java