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https://issues.apache.org/jira/browse/HDFS-11096?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15799802#comment-15799802
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Sean Mackrory commented on HDFS-11096:
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I've been doing a lot of testing. I've posted some automation here, we may want
to hook into a Jenkins job or something:
https://github.com/mackrorysd/hadoop-compatibility. I've tested running a bunch
of MapReduce jobs while doing a rolling upgrade of HDFS, and haven't had any
failures that indicate an incompatibility. I've also tested pulling data from
an old cluster onto a new cluster. I'll keep adding other aspects to the tests
to improve coverage.
I haven't seen a way to whitelist stuff. Filed an issue with jacc:
https://github.com/lvc/japi-compliance-checker/issues/36.
As for the incompatibilities, I think there's relatively action to be taken, so
I'll file JIRAs for those. In detail: metrics and s3a are technically violating
the contract, but in all cases it would be some serious baggage and due to
their nature I think it's acceptable. I think SortedMapWritable should be put
back but deprecated (I'm sure someone's depending on it somewhere and it should
be trivial), and FileStatus should still implement Comparable. Not so sure
about NameodeMXBean, the missing configuration keys, or the cases of reduced
visibility. I'm inclined to leave these as-is unless we know it breaks
something and they care. They are technically incompatibilities, so maybe
someone else feels differently (or is aware of applications they are likely to
break), but it would be nice to shed baggage and poor practices where we can.
All other issues I feel more confident that they're either not actually
breaking the contract or are extremely unlikely to break anything enough to
warrant sticking with the old way. I'll sleep on some of these one more night
and file JIRAs to start addressing the issues I think are important enough
tomorrow.
> 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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