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https://issues.apache.org/jira/browse/HDFS-6800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14107286#comment-14107286
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James Thomas commented on HDFS-6800:
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[~arpitagarwal], thanks for the response. The reason we use the flag in all
cases is that we now support rolling upgrade rollbacks to an old DataNode
layout version (a process that in the non-rolling upgrade case requires this
flag), so I thought it would be easiest to just have administrators use the
flag in all rollback cases (as you can see in the patch, there is no
performance overhead if the rollback is not to a previous DataNode layout
version). It's really just a convention, but I think it is simple and makes
sense.
As for the testing, as [~cmccabe] mentioned in his first comment above, I have
already run tests with rolling upgrade to a new DN layout version with 100k
blocks, and the hard link time was very fast. I don't think restoring the
previous directory during a rollback is very expensive -- the only possible
problem I see is the deletion of the current dir. Is that what you were
concerned about? If so, what is the budget for rollback time? I can't imagine
the deletion would take more than a couple tens of seconds.
> 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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