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From "Wei-Chiu Chuang (Jira)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-14821) Make HDFS-14617 (fsimage sub-sections) off by default
Date Wed, 04 Sep 2019 17:46:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-14821?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16922708#comment-16922708
] 

Wei-Chiu Chuang commented on HDFS-14821:
----------------------------------------

+1 best to avoid the complication of rolling upgrades/downgrades. We've seen how it played
out with EC (HDFS-13596/HDFS-14396)

> Make HDFS-14617 (fsimage sub-sections) off by default
> -----------------------------------------------------
>
>                 Key: HDFS-14821
>                 URL: https://issues.apache.org/jira/browse/HDFS-14821
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: namenode
>    Affects Versions: 3.3.0
>            Reporter: Stephen O'Donnell
>            Assignee: Stephen O'Donnell
>            Priority: Blocker
>
> In HDFS-14617 I incorrectly stated that a fsimage with sub-sections listed in the image
summary section could be loaded without the HDFS-14617 patch applied. However that was not
correct. If the cluster was upgraded to a version containing this feature, then upon downgrade
the image will fail to load.
> I believe the simplest solution to this problem is to have this feature disabled by default.
That way, after upgrading to a version with the feature, the operator can make a decision
to enable the feature. If they later want to downgrade, then they must:
>  # Disable the feature
>  # Save the namespace, which will create an image without sub-sections
>  # Perform the downgrade.
> Even though the steps to downgrade are simple, we cannot expect people to be aware of
this, and hence it is safest to disable the feature by default.
> The only alternative is to create a new image layout version, but that seems excessive
for this feature, as the core contents of the image have not changed and the sub-sections
can be removed by disabling the feature and saving the namespace.



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