@Override
    public Map<byte[], List<byte[]>> rollWriter(boolean force)
      throws FailedLogCloseException, IOException {
      Map<byte[], List<byte[]>> regions = super.rollWriter(force);
      rolls.getAndIncrement();
      return regions;
    }
Testing sync/append failures. Copied from TestHRegion.
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestFailedAppendAndSync.java