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https://issues.apache.org/jira/browse/HDFS-1612?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12992386#comment-12992386
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dhruba borthakur commented on HDFS-1612:
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This portion of the HDFS document is outdated. would you like to submit a patch
that brings it upto date?
> HDFS Design Documentation is outdated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: 0.20.2, 0.21.0
> 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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