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From "Tsz Wo (Nicholas), SZE (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1612) HDFS Design Documentation is outdated
Date Thu, 24 Feb 2011 18:20:39 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1612?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12998980#comment-12998980
] 

Tsz Wo (Nicholas), SZE commented on HDFS-1612:
----------------------------------------------

> It would be great if you can update the doc. See also HDFS-1612.
I mean to say HDFS-1388 but not HDFS-1612.

> HDFS Design Documentation is outdated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
>            Assignee: Joe Crobak
>            Priority: Minor
>         Attachments: HDFS-1612.patch
>
>
> 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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