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https://issues.apache.org/jira/browse/HDFS-957?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12832216#action_12832216
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Todd Lipcon commented on HDFS-957:
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See my motivation for the ability to tell if an image is complete here:

https://issues.apache.org/jira/browse/HDFS-955?focusedCommentId=12832212&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_12832212

> FSImage layout version should be only once file is complete
> -----------------------------------------------------------
>
>                 Key: HDFS-957
>                 URL: https://issues.apache.org/jira/browse/HDFS-957
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>    Affects Versions: 0.22.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: hdfs-957.txt
>
>
> Right now, the FSImage save code writes the LAYOUT_VERSION at the head of the 
> file, along with some other headers, and then dumps the directory into the 
> file. Instead, it should write a special IMAGE_IN_PROGRESS entry for the 
> layout version, dump all of the data, then seek back to the head of the file 
> to write the proper LAYOUT_VERSION. This would make it very easy to detect 
> the case where the FSImage save got interrupted.

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