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https://issues.apache.org/jira/browse/HDFS-5535?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13912136#comment-13912136
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Tsz Wo (Nicholas), SZE commented on HDFS-5535:
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> It looks like this feature is getting close, nice work! Can we get a rev of 
> the design doc at a less-high level as we approach merge? It seems like 
> details surrounding e.g. user API and implementation have been ironed out, so 
> should be included. ...

Thanks.  We are going to revise the design doc.  However, there is not much 
user API to add since this feature, Rolling Upgrade, does not really have 
meaningful user API.  There are new CLI and startup options.  The usage of them 
and other user guide can be found in the user documentation committed to the 
branch.

> ..., sorry in advance if these were already answered in the comments:

No problem.  Perhaps, you should take your time and read the comments first.  
No hurry.

> Can you expand on NN/DN consistency with the rollback marker and heartbeat 
> notifications? I'm not familiar with append or lease recovery, so it'd be 
> nice to get more explanation on those in particular.

This part is similar to current upgrade/rollback mechanism.  Do you familiar 
with it?  For append and lease recovery, you may want to read the append design 
doc in HDFS-265.

> Umbrella jira for improved HDFS rolling upgrades
> ------------------------------------------------
>
>                 Key: HDFS-5535
>                 URL: https://issues.apache.org/jira/browse/HDFS-5535
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: datanode, ha, hdfs-client, namenode
>    Affects Versions: 3.0.0, 2.2.0
>            Reporter: Nathan Roberts
>         Attachments: HDFSRollingUpgradesHighLevelDesign.pdf, 
> h5535_20140219.patch, h5535_20140220-1554.patch, h5535_20140220b.patch, 
> h5535_20140221-2031.patch, h5535_20140224-1931.patch, 
> h5535_20140225-1225.patch
>
>
> In order to roll a new HDFS release through a large cluster quickly and 
> safely, a few enhancements are needed in HDFS. An initial High level design 
> document will be attached to this jira, and sub-jiras will itemize the 
> individual tasks.



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