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.

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。

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。