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https://issues.apache.org/jira/browse/HADOOP-5724?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12708516#action_12708516
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Hairong Kuang commented on HADOOP-5724:
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> any reason why the NN does not remove a replica from the block map at the
> time of sending the block-delete request to the datanode when it is trying to
> reduce over-replication?
One more reason: because there is a delay between scheduling a replica to
remove and the replica getting removed at a datanode, keeping the replica in
blocksMap could avoid placing the block on this datanode if ever an
under-replication happens to occur. That why we had HADOOP-4643 instead of
HADOOP-4540.
> Datanode should report deletion of blocks to Namenode explicitly
> ----------------------------------------------------------------
>
> Key: HADOOP-5724
> URL: https://issues.apache.org/jira/browse/HADOOP-5724
> Project: Hadoop Core
> Issue Type: Bug
> Reporter: Suresh Srinivas
> Assignee: Suresh Srinivas
> Fix For: 0.21.0
>
> Attachments: blockdel.patch, blockdel.patch
>
>
> Currently datanode notifies namenode newly added blocks and the blocks that
> are corrupt. There is no explicit message from the datanode to the namenode
> to indicate the deletion of blocks. Block reports from the datanode is the
> only way for the namenode to learn about the deletion of blocks at a
> datanode. With the addition of explicit request to indicate to block
> deletion, block report interval (which is currently 1 hour) can be increased
> to a longer duration. This reduces load on both namenode and datanodes.
>
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