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From "Joe Crobak (JIRA)" <j...@apache.org>
Subject [jira] Created: (HDFS-1612) HDFS Design Documentation is outdated
Date Mon, 07 Feb 2011 15:54:31 GMT
HDFS Design Documentation is outdated
-------------------------------------

                 Key: HDFS-1612
                 URL: https://issues.apache.org/jira/browse/HDFS-1612
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: documentation
    Affects Versions: 0.21.0, 0.20.2
         Environment: http://hadoop.apache.org/hdfs/docs/current/hdfs_design.html#The+Persistence+of+File+System+Metadata
http://hadoop.apache.org/common/docs/r0.20.2/hdfs_design.html#The+Persistence+of+File+System+Metadata
            Reporter: Joe Crobak
            Priority: Minor


I was trying to discover details about the Secondary NameNode, and came across the description
below in the HDFS design doc.

{quote}
The NameNode keeps an image of the entire file system namespace and file Blockmap in memory.
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. When the NameNode starts up, it
reads the FsImage and EditLog from disk, applies all the transactions from the EditLog to
the in-memory representation of the FsImage, and flushes out this new version into a new FsImage
on disk. It can then truncate the old EditLog because its transactions have been applied to
the persistent FsImage. This process is called a checkpoint. *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.*
{quote}
(emphasis mine).

Note that this directly conflicts with information in the hdfs user guide, http://hadoop.apache.org/common/docs/r0.20.2/hdfs_user_guide.html#Secondary+NameNode
and http://hadoop.apache.org/hdfs/docs/current/hdfs_user_guide.html#Checkpoint+Node

I haven't done a thorough audit of that doc-- I only noticed the above inaccuracy.


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