@Override
  public void writeToDisk() {
    if (diskRegion == null) {
      DataPolicy dp = getDataPolicy();
      if (dp.isEmpty()) {
        throw new IllegalStateException(
            String.format("Cannot write a region with data-policy %s to disk.",
                dp));
      }
      if (!dp.withPersistence() && !isOverflowEnabled()) {
        throw new IllegalStateException(
            "Cannot write a region that is not configured to access disks.");
      }
    } else {
      diskRegion.asynchForceFlush();
    }
  }
Lets the customer do an explicit evict of a value to disk and removes the value from memory.
geode-core/src/main/java/org/apache/geode/internal/cache/LocalRegion.java