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https://issues.apache.org/jira/browse/HDFS-6800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14107133#comment-14107133
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James Thomas commented on HDFS-6800:
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[~cmccabe], thanks for checking this out. Why couldn't we simply do the delete
of the trash dir before the doRollback() call? If the trash removal succeeds
and the doRollback() fails, it's not a problem, because we don't need the trash
at all in the case where a previous directory is created. After this happens,
we may decide to 1) finalize the upgrade instead of rolling it back, in which
case we'll simply delete the previous directory and move forward or 2) make a
second attempt at the rollback, in which case we'll attempt to delete the
nonexistent trash directory (not a problem) and then restore the previous
directory in doRollback(). So we're fine in either case.
[~arpitagarwal], sorry to keep bothering you, but I was hoping to get the work
committed by next week (when my summer internship ends), so it'd be great if
you could take a look or direct me to someone else who might be qualified.
> Determine how Datanode layout changes should interact with rolling upgrade
> --------------------------------------------------------------------------
>
> Key: HDFS-6800
> URL: https://issues.apache.org/jira/browse/HDFS-6800
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: datanode
> Affects Versions: 2.6.0
> Reporter: Colin Patrick McCabe
> Assignee: James Thomas
> Attachments: HDFS-6800.2.patch, HDFS-6800.3.patch, HDFS-6800.4.patch,
> HDFS-6800.patch
>
>
> We need to handle attempts to rolling-upgrade the DataNode to a new storage
> directory layout.
> One approach is to disallow such upgrades. If we choose this approach, we
> should make sure that the system administrator gets a helpful error message
> and a clean failure when trying to use rolling upgrade to a version that
> doesn't support it. Based on the compatibility guarantees described in
> HDFS-5535, this would mean that *any* future DataNode layout changes would
> require a major version upgrade.
> Another approach would be to support rolling upgrade from an old DN storage
> layout to a new layout. This approach requires us to change our
> documentation to explain to users that they should supply the {{\-rollback}}
> command on the command-line when re-starting the DataNodes during rolling
> rollback. Currently the documentation just says to restart the DataNode
> normally.
> Another issue here is that the DataNode's usage message describes rollback
> options that no longer exist. The help text says that the DN supports
> {{\-rollingupgrade rollback}}, but this option was removed by HDFS-6005.
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