public static void doAsyncFlush(VersionTag tag, InternalRegion region) {
      if (region.isThisRegionBeingClosedOrDestroyed()) {
        return;
      }
      DiskRegion dr = region.getDiskRegion();
      if (!dr.isBackup()) {
        return;
      }
      assert !dr.isSync();
      dr.acquireReadLock();
      try {
        dr.getDiskStore().putVersionTagOnly(region, tag, true);
      } finally {
        dr.releaseReadLock();
      }
    }
Writes the value of this {@code DiskEntry} to disk and {@code null} s out the reference to
the value to free up VM space.
<p>
Note that if the value had already been written to disk, it is not written again.
<p>
Caller must synchronize on entry and it is assumed the entry is evicted
geode-core/src/main/java/org/apache/geode/internal/cache/entries/DiskEntry.java