Konstantin Shvachko commented on HDFS-9500:

Sounds like the condition for decrementing the old version is not accurate. I 
see this on trunk and other versions.
[~yangzhe1991] do you still plan to work on it?

> datanodesSoftwareVersions map may counting wrong when rolling upgrade
> ---------------------------------------------------------------------
>                 Key: HDFS-9500
>                 URL: https://issues.apache.org/jira/browse/HDFS-9500
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 2.7.1, 2.6.2
>            Reporter: Phil Yang
>            Assignee: Phil Yang
>         Attachments: 9500-v1.patch
> While rolling upgrading, namenode's website overview will report there are 
> two versions datanodes in the cluster, for example, 2.6.0 has x nodes and 
> 2.6.2 has y nodes. However, sometimes when I stop a datanode in old version 
> and start a new version one, namenode only increases the number of new 
> version but not decreases the number of old version. So the total number x+y 
> will be larger than the number of datanodes. Even all datanodes are upgraded, 
> there will still have the messages that there are several datanode in old 
> version. And I must run hdfs dfsadmin -refreshNodes to clear this message.
> I think this issue is caused by DatanodeManager.registerDatanode. If nodeS in 
> old version is not alive because of shutting down, it will not pass 
> shouldCountVersion, so the number of old version won't be decreased. But this 
> method only judges the status of heartbeat and isAlive at that moment, if 
> namenode has not removed this node which will decrease the version map and 
> this node restarts in the new version, the decrementVersionCount belongs to 
> this node will never be executed.
> So the simplest way to fix this is that we always recounting the version map 
> in registerDatanode since it is not a heavy operation.

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