Hadoop HDFS源码分析 关于数据块的类

1.BlocksMap

官方代码中的注释为：

/**
 * This class maintains the map from a block to its metadata.
 * block‘s metadata currently includes blockCollection it belongs to and
 * the datanodes that store the block.
 */

　　BlocksMap数据块映射，管理名字节点上的数据块的元数据。数据块的元数据包括数据块所属的INode和那些数据节点保存数据块等信息。也就是定位哪个数据块在哪个数据节点上。

2.DatanodeDescriptor

DatanodeDescriptor数据节点描述符。官方解释为：

/**
 * This class extends the DatanodeInfo class with ephemeral information (eg
 * health, capacity, what blocks are associated with the Datanode) that is
 * private to the Namenode, ie this class is not exposed to clients.
 */

是名字节点中对数据节点的抽象，继承自DatanodeInfo。

3.BlockInfo

BlockInfo 数据块信息，是Block的子类，它增加了数据块所属的INode信息和保存该数据块的数据节点信息。

/**
 * BlockInfo class maintains for a given block
 * the {@link INodeFile} it is part of and datanodes where the replicas of
 * the block are stored.
 * BlockInfo class maintains for a given block
 * the {@link BlockCollection} it is part of and datanodes where the replicas of
 * the block are stored.
 */

数据块所在数据节点的信息保存在Object[] triplets。