private static void doSyncOfUnflushedWALChanges(final WAL wal, final RegionInfo hri)
    throws IOException {
    if (wal == null) {
      return;
    }
    try {
      wal.sync(); // ensure that flush marker is sync'ed
    } catch (IOException ioe) {
      wal.abortCacheFlush(hri.getEncodedNameAsBytes());
      throw ioe;
    }
  }
Sync unflushed WAL changes. See HBASE-8208 for details
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegion.java