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https://issues.apache.org/jira/browse/HDFS-1521?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Todd Lipcon updated HDFS-1521:
------------------------------

    Attachment: hdfs-1521.3.txt

This patch switches to logging a txid for every edit and verifying strict 
sequential ordering on load.

I also left the txid in the header - it seemed to me this is advantageous just 
as something that *must* be there at the top of every edit file. If others 
disagree we can take it out.

Added some basic tests as well to ensure we can still read the old format.

> Persist transaction ID on disk between NN restarts
> --------------------------------------------------
>
>                 Key: HDFS-1521
>                 URL: https://issues.apache.org/jira/browse/HDFS-1521
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>    Affects Versions: 0.22.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>             Fix For: 0.22.0
>
>         Attachments: hdfs-1521.3.txt, hdfs-1521.txt, hdfs-1521.txt
>
>
> For HDFS-1073 and other future work, we'd like to have the concept of a 
> transaction ID that is persisted on disk with the image/edits. We already 
> have this concept in the NameNode but it resets to 0 on restart. We can also 
> use this txid to replace the _checkpointTime_ field, I believe.

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