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https://issues.apache.org/jira/browse/HDFS-1612?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12994148#comment-12994148
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Joe Crobak commented on HDFS-1612:
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{quote}
This portion of the HDFS document is outdated. would you like to submit a patch
that brings it upto date?
{quote}
I'd be happy to create a patch. I did a quick look over the hdfs design doc,
and there seem to be a few new features of 0.21 (e.g. append, symbolic links)
as well as some older features (e.g. quoatas) that aren't documented correctly.
I'd be happy to update those, too, either as part of this patch or separately.
> HDFS Design Documentation is outdated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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