分析NameNode/ DataNode/ SecondaryNameNode源码注释

 /********************************************************** * NameNode serves as both directory namespace manager and
    
  * "inode table" for the Hadoop DFS.  There is a single NameNode
    
  * running in any DFS deployment.  (Well, except when there
    
  * is a second backup/failover NameNode, or when using federated NameNodes.)
    
  * * The NameNode controls two critical tables:
    
  *   1)  filename->blocksequence (namespace)
    
  *   2)  block->machinelist ("inodes")
    
  * * The first table is stored on disk and is very precious.
    
  * The secon