@Override
  protected void doSync(long txid, boolean forceSync) throws IOException {
    if (this.ft == FailureType.SYNC) {
      throw new IOException("sync");
    }
    super.doSync(txid, forceSync);
  }
This is a utility class, used by tests, which fails operation specified by FailureType enum
hbase-server/src/test/java/org/apache/hadoop/hbase/wal/FaultyFSLog.java