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https://issues.apache.org/jira/browse/HDFS-11096?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15802380#comment-15802380
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Sean Mackrory commented on HDFS-11096:
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In the SortedMapWritable case, the class is actually still there in the same
package, but it's now templated slightly differently to avoid a warning. This
actually did cause a source incompatibility in Hive and was removed from
branch-2. Not sure if this would cause problems at runtime if an application
with the old version was deployed on the new version. Will look into it.
Details in HADOOP-13338.
FileStatus also still implements Comparable but specifies it's own type. It
requires a FileStatus, not an Object, although it used to cast the Object to a
FileStatus anyway. So the only this would actually fail is if someone was
casting a FileStatus to an Object before the call to compare(). So it could
happen, but it's unlikely, and fixing that is something someone could do before
upgrading to Hadoop 3. That change was in
https://issues.apache.org/jira/browse/HADOOP-12209. I'm inclined to fix that by
documentating that we support rolling upgrade, assuming a set of things have
been checked, including that.
> Support rolling upgrade between 2.x and 3.x
> -------------------------------------------
>
> Key: HDFS-11096
> URL: https://issues.apache.org/jira/browse/HDFS-11096
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: rolling upgrades
> Affects Versions: 3.0.0-alpha1
> Reporter: Andrew Wang
> Priority: Blocker
>
> trunk has a minimum software version of 3.0.0-alpha1. This means we can't
> rolling upgrade between branch-2 and trunk.
> This is a showstopper for large deployments. Unless there are very compelling
> reasons to break compatibility, let's restore the ability to rolling upgrade
> to 3.x releases.
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