The advantage of replication is that reads are always local, as everybody has the data, and there is no re-balancing going on when a new node joins or an existing node leaves the cluster.

这种复制的优点在于它通常只在本地节点读取数据，因为每个节点都拥有这些数据；另外，当群集中有新节点加入或者需要移除现存节点的时候，它也不需要重新进行负载均衡。

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In asynchronous mode, the write transaction is acknowledged after the write data is stored on the local node's storage; the replication of the data to the peer node occurs in the background.

在异步模式下，写数据被存储到本地节点存储中之后，写事务被认可；对等节点中的数据的副本出现在后台中。

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When a client is writing data to an HDFS file, its data is first written to a local file as explained in the previous section. Suppose the HDFS file has a replication factor of three.

流水线复制当某个客户端向HDFS文件写数据的时候，一开始是写入本地临时文件，假设该文件的replication因子设置为3，那么客户端会从Namenode获取一张Datanode列表来存放副本。

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