@Override
    protected Set<String> initialValue() {
      return new HashSet<String>();
    }
An implementation of the StoreFlusher. It extends the DefaultStoreFlusher. If the store is not a
mob store, the flusher flushes the MemStore the same with DefaultStoreFlusher, If the store is a
mob store, the flusher flushes the MemStore into two places. One is the store files of HBase, the
other is the mob files.
<ol>
<li>Cells that are not PUT type or have the delete mark will be directly flushed to HBase.</li>
<li>If the size of a cell value is larger than a threshold, it'll be flushed to a mob file,
another cell with the path of this file will be flushed to HBase.</li>
<li>If the size of a cell value is smaller than or equal with a threshold, it'll be flushed to
HBase directly.</li>
</ol>
hbase-server/src/main/java/org/apache/hadoop/hbase/mob/DefaultMobStoreFlusher.java