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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7646) HDFS truncate may remove data from the supposedly read-only previous/ directory during an upgrade
Date Wed, 21 Jan 2015 04:05:35 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7646?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14285130#comment-14285130
] 

Colin Patrick McCabe commented on HDFS-7646:
--------------------------------------------

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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