@Override
  protected void shutdown0() throws IOException {
    // save the last exception and rethrow
    IOException failure = null;
    for (WALProvider provider : cached.values()) {
      try {
        provider.shutdown();
      } catch (IOException e) {
        LOG.error("Problem shutting down wal provider '" + provider + "': " + e.getMessage());
        if (LOG.isDebugEnabled()) {
          LOG.debug("Details of problem shutting down wal provider '" + provider + "'", e);
        }
        failure = e;
      }
    }
    if (failure != null) {
      throw failure;
    }
  }
A group-provider mapping, make sure one-one rather than many-one mapping
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/RegionGroupingProvider.java