Hadoop: The Definitive Guide, 3rd Edition

Chapter 10. Administering Hadoop

The previous chapter was devoted to setting up a Hadoop cluster. In this chapter, we look at the procedures to keep a cluster running smoothly.

HDFS

Persistent Data Structures

As an administrator, it is invaluable to have a basic understanding of how the components of HDFS—the namenode, the secondary namenode, and the datanodes—organize their persistent data on disk. Knowing which files are which can help you diagnose problems or spot that something is awry.

Namenode directory structure

A newly formatted namenode creates the following directory structure:

${dfs.name.dir}/ └── current/     ├── VERSION     ├── edits     ├── fsimage     └── fstime         

Recall from Chapter 9 that the dfs.name.dir property is a list of directories, with the same contents mirrored in each directory. This mechanism provides resilience, particularly if one of the directories is an NFS mount, as is recommended.

The VERSION file is a Java properties file that contains information about the version of HDFS that is running. Here are the contents of a typical file:

#Tue Mar 10 19:21:36 GMT 2009
namespaceID=134368441
cTime=0
storageType=NAME_NODE
layoutVersion=-18

The layoutVersion is a negative integer that defines the version of HDFS’s persistent data structures. This version number has no relation to the release number of the Hadoop distribution. Whenever the layout changes, the version number is decremented (for example, the version after −18 is −19). When this happens, HDFS needs ...