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。
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。
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。
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。
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。
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。
For this reason, it is important to make the namenode resilient to failure, and Hadoop provides two mechanisms for this.
基于这个原因，确保namenode对故障的弹性机制很重要，为此，hadoop提供了两种机制。
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。
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。
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。
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secondarynamenode，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。
This key metadata item is designed to be compact, such that a NameNode with 4 GB of RAM is plenty to support a huge number of files and directories.
这个关键的元数据设计得很紧凑，因而一个带有4G内存的Namenode足够支撑海量的文件和目录。
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。
On startup, the NameNode enters a special state called Safemode. Replication of data blocks does not occur when the NameNode is in the Safemode state.
SafeModeNamenode启动后会进入一个称为SafeMode的特殊状态，处在这个状态的Namenode是不会进行数据块的复制的。
In the current implementation, a checkpoint only occurs when the NameNode starts up. Work is in progress to support periodic checkpointing in the near future.
在当前实现中，checkpoint只发生在Namenode启动时，在不久的将来我们将实现支持周期性的checkpoint。
When a file is closed, the remaining un-flushed data in the temporary local file is transferred to the DataNode. The client then tells the NameNode that the file is closed.
当文件关闭时，在临时文件中剩余的没有flush的数据也会传输到指定的Datanode，然后客户端告诉Namenode文件已经关闭。
They store and retrieve blocks when they are told to (by clients or the namenode), and they report back to the namenode periodically with lists of blocks that they are storing.
它们存储和检索数据块(受客户端或者namenode通知)，并且周期性的向namenode报告它们所存储的块列表信息。
The NameNode detects this condition by the absence of a Heartbeat message. The NameNode marks DataNodes without recent Heartbeats as dead and does not forward any new IO requests to them.
Namenode通过心跳包的缺失检测到这一情况，并将这些Datanode标记为dead，不会将新的IO请求发给它们。
The NameNode detects this condition by the absence of a Heartbeat message. The NameNode marks DataNodes without recent Heartbeats as dead and does not forward any new IO requests to them.
Namenode通过心跳包的缺失检测到这一情况，并将这些Datanode标记为dead，不会将新的IO请求发给它们。
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