June 2009
Intermediate to advanced
524 pages
16h 12m
English
Content preview from Hadoop: The Definitive GuideBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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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 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.