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https://issues.apache.org/jira/browse/HDFS-17227?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Shuyan Zhang reassigned HDFS-17227:
-----------------------------------
Assignee: Shuyan Zhang
> EC: Fix bug in choosing targets when racks is not enough.
> ---------------------------------------------------------
>
> Key: HDFS-17227
> URL: https://issues.apache.org/jira/browse/HDFS-17227
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Shuyan Zhang
> Assignee: Shuyan Zhang
> Priority: Major
>
> *Bug description*
> If,
> 1. There is a striped block blockinfo1, which has an excess replica on
> datanodeA.
> 2. blockinfo1 has an internal block that needs to be reconstruction.
> 3. The number of racks is less than the number of internal blocks of
> Blockinfo1.
> Then, NN may choose datanodeA to reconstruct the internal block, resulting in
> two internal blocks of blockinfo1 on datanodeA, causing confusion.
> *Root cause and solution*
> When we use `BlockPlacementPolicyRackFaultTolerant` for choosing targets and
> the racks is insufficient, `chooseEvenlyFromRemainingRacks` will be called.
> Currently, `chooseEvenlyFromRemainingRacks` calls `chooseOnce`, `chooseOnce`
> use `newExcludeNodes` as parameter instead of `excludedNodes`. When we choose
> targets for reconstructing internal blocks, 'newExcludeNodes' only includes
> those datanodes that contain live replicas, and does not include datanodes
> that have excess replicas. This may result in datanodes with excess replicas
> is chosen.
> I don't think we need to use 'newExcludeNodes', just pass `excludedNodes` to
> `chooseOnce`.
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