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https://issues.apache.org/jira/browse/HDFS-7646?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14285130#comment-14285130
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Colin Patrick McCabe commented on HDFS-7646:
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Yes, you're right. Sorry. I can see that {{shouldCopyOnTruncate}} will return
true if the upgrade has not yet been finalized.
> HDFS truncate may remove data from the supposedly read-only previous/
> directory during an upgrade
> -------------------------------------------------------------------------------------------------
>
> Key: HDFS-7646
> URL: https://issues.apache.org/jira/browse/HDFS-7646
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Colin Patrick McCabe
> Priority: Blocker
>
> During a DataNode layout version upgrade, HDFS creates hardlinks. These
> hardlinks allow the same block to be accessible from both the current/ and
> previous/ directories. Rollback is possible by deleting the current/
> directory and renaming previous/ to current/.
> However, if the user truncates one of these hardlinked block files, it
> effectively eliminates the ability to roll back to the previous data.
> We probably need to disable truncation-in-place during a DataNode upgrade.
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