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https://issues.apache.org/jira/browse/HDFS-6800?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14115878#comment-14115878
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James Thomas commented on HDFS-6800:
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Thanks, Arpit. Here are the key issues I think we need to test (beyond simply
checking that rolling upgrade and rolling upgrade rollback work without data
loss after HDFS-6482):
For a rolling upgrade that changes the DN layout version:
* On upgrade finalize, make sure that the {{previous}} directories on the DNs
are deleted
* On upgrade rollback, make sure the trash is deleted and not restored
* On upgrade rollback, make sure the {{previous}} directories on the DNs are
not deleted prior to the rollback (need to wait until a few block reports have
occurred to make sure there is no deletion)
For a rolling upgrade that does not change the DN layout version:
* Make sure that we roll back correctly using the {{-rollback}} flag
Downgrade is not a concern for us since we cannot downgrade after changing the
DN layout version.
> Support Datanode layout changes 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
> Fix For: 3.0.0, 2.6.0
>
> Attachments: HDFS-6800.2.patch, HDFS-6800.3.patch, HDFS-6800.4.patch,
> HDFS-6800.5.patch, HDFS-6800.6.patch, HDFS-6800.7.patch, HDFS-6800.8.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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