es底层lucene保证indexwrite和translog的数据一致性，为什么还要采用2PC机制？

通过es的lucene源码中看到有一个TwoPhaseCommit接口，接口的实现类有两个：IndexWrite和Translog，

public interface TwoPhaseCommit {



  /**

   * The first stage of a 2-phase commit. Implementations should do as much work

   * as possible in this method, but avoid actual committing changes. If the

   * 2-phase commit fails, {@link #rollback()} is called to discard all changes

   * since last successful commit.

   */

  public long prepareCommit() throws IOException;



  /**

   * The second phase of a 2-phase commit. Implementations should ideally do

   * very little work in this method (following {@link #prepareCommit()}, and

   * after it returns, the caller can assume that the changes were successfully

   * committed to the underlying storage.

   */

  public long commit() throws IOException;



  /**

   * Discards any changes that have occurred since the last commit. In a 2-phase

   * commit algorithm, where one of the objects failed to {@link #commit()} or

   * {@link #prepareCommit()}, this method is used to roll all other objects

   * back to their previous state.

   */

  public void rollback() throws IOException;

}

我记得es的一致性性是采用类似raft协议的选举算法实现的，为什么这里还要用的两段式提交，2PC效率低，因为是同步阻塞，必须全部成功，