NameNode

Once the MapReduce operation for a particular search key is completed, the NameNode returns the output value to the server and in turn to the client.

完成特定的搜索关键字MapReduce操作之后，NameNode将输出值返回到服务器并交付客户端。

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First, request the namenode to format the DFS file system.

首先，请求namenode对DFS文件系统进行格式化。

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The NameNode uses a transaction log called the EditLog to persistently record every change that occurs to file system metadata.

对于任何对文件元数据产生修改的操作，Namenode都使用一个称为Editlog的事务日志记录下来。

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The existence of a single NameNode in a cluster greatly simplifies the architecture of the system.

单一节点的Namenode大大简化了系统的架构。

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Loads are compared by the NameNode and then the details about the free-est neighbor nodes are sent to the specific DataNode.

NameNode比较负载，然后将有关最空闲相邻节点的详细信息发送到特定的DataNode。

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The namenode is the master server in Hadoop and manages the file system namespace and access to the files stored in the cluster.

namenode是Hadoop中的主服务器，它管理文件系统名称空间和对集群中存储的文件的访问。

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Recall that at the top of the Hadoop cluster is the namenode, which manages the HDFS.

位于Hadoop集群最上层的是namenode，它管理HDFS。

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This synchronous updating of multiple copies of the FsImage and EditLog may degrade the rate of namespace transactions per second that a NameNode can support.

这个同步操作可能会降低Namenode每秒能支持处理的namespace事务。

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The NameNode executes file system namespace operations like opening, closing, and renaming files and directories.

Namenode执行文件系统的namespace操作，例如打开、关闭、重命名文件和目录，同时决定block到具体Datanode节点的映射。

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There's also a secondary namenode, which isn't a redundant daemon for the namenode but instead provides period checkpointing and housekeeping tasks.

还有一个secondary namenode，它不是namenode的冗余守护进程，而是提供周期检查点和清理任务。

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They also respond to commands that create, delete, and replicate blocks received from the NameNode.

它们还响应从NameNode接收的块创建、删除和复制命令。

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You can perform a couple of tests to ensure that Hadoop is up and running normally ( at least the namenode).

可以通过几个检查确认Hadoop（至少是namenode）已经启动并正常运行。

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When the load increases in a DataNode to the threshold level, it sends a request to the NameNode.

当DataNode的负载增加到阈值级别时，它将向该NameNode发送一个请求。

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A network partition can cause a subset of DataNodes to lose connectivity with the NameNode.

网络切割可能导致一部分Datanode跟Namenode失去联系。

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But, due to the characteristics of distributed storage, there are" NameNode "and" DataNode," which take each of their responsibility.

但是，由于分布式存储的性质，有“NameNode”和“DataNode”的概念，它们承担各自的责任。

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From this, you can see that the namenode is up and able to service the local namespace.

可以看出namenode已经启动，能够为本地名称空间提供服务。

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You'll find one namenode and one secondary namenode in a Hadoop cluster.

在每个Hadoop集群中可以找到一个namenode和一个secondary namenode。

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The Datanodes also perform block creation, deletion, and replication upon instruction from the Namenode.

数据节点执行来自于名字节点的建立 、 删除 、 复制指令.

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At this point, the NameNode commits the file creation operation into a persistent store.

此时Namenode才将文件创建操作提交到持久存储。

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An HDFS cluster consists of a single NameNode, a master server that manages the file system namespace and regulates access to files by clients.

一个HDFS集群是有一个Namenode和一定数目的Datanode组成。Namenode是一个中心服务器，负责管理文件系统的namespace和客户端对文件的访问。

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The NameNode had information about the load levels of the specific DataNode's nearest neighbors.

NameNode获得特定DataNode最邻近节点的负载级别信息。

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For this reason, the NameNode can be configured to support maintaining multiple copies of the FsImage and EditLog.

因而，Namenode可以配置成支持维护多个FsImage和Editlog的拷贝。

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