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https://issues.apache.org/jira/browse/HDFS-7646?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14284897#comment-14284897
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Konstantin Shvachko commented on HDFS-7646:
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In place truncate is disabled during upgrades. NN will do copy on truncate
until upgrade is finalized or rolled back. This is reflected in the design doc
of HDFS-3107, and there is a unit test verifying that.
> 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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