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From "Chao Sun (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-14067) Allow manual failover between standby and observer
Date Tue, 13 Nov 2018 23:04:00 GMT

    [ https://issues.apache.org/jira/browse/HDFS-14067?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16685870#comment-16685870

Chao Sun commented on HDFS-14067:

Thanks for the clarification [~xkrogen]. I think the check was introduced HADOOP-8247, and
the reason was because if we allow manual and auto failover at the same time, the state tracked
by ZK could be inconsistent with the actual state. For instance, in a HA cluster with 1 active
and 1 standby, potentially one could manually transition the state of the NNs and swap their
roles, but the ZK will still track the old state.

In our case, I do see some potential inconsistency. Currently the ZKFailoverController only
tracks active and standby state. And, if we manually transition a standby to observer, ZK
will still keep the old state and even make the observer eligible for election, which is incorrect.
This somewhat overlaps with HDFS-13182.

> Allow manual failover between standby and observer
> --------------------------------------------------
>                 Key: HDFS-14067
>                 URL: https://issues.apache.org/jira/browse/HDFS-14067
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>            Reporter: Chao Sun
>            Assignee: Chao Sun
>            Priority: Major
>         Attachments: HDFS-14067-HDFS-12943.000.patch
> Currently if automatic failover is enabled in a HA environment, transition from standby
to observer would be blocked:
> {code}
> [hdfs@*** hadoop-3.3.0-SNAPSHOT]$ bin/hdfs haadmin -transitionToObserver ha2
> Automatic failover is enabled for NameNode at ****
> Refusing to manually manage HA state, since it may cause
> a split-brain scenario or other incorrect state.
> If you are very sure you know what you are doing, please
> specify the --forcemanual flag.
> {code}
> We should allow manual transition between standby and observer in this case.

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