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https://issues.apache.org/jira/browse/HADOOP-3201?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12588178#action_12588178
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dhruba borthakur commented on HADOOP-3201:
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+1 for Option 1.
> namenode should be able to retrieve block metadata from a datanode
> ------------------------------------------------------------------
>
> Key: HADOOP-3201
> URL: https://issues.apache.org/jira/browse/HADOOP-3201
> Project: Hadoop Core
> Issue Type: New Feature
> Components: dfs
> Reporter: dhruba borthakur
> Assignee: Tsz Wo (Nicholas), SZE
>
> The Append design (HADOOP-1700) requires that the namenode be able to contact
> the datanode when a lease expires.
> When a lease expires, the namenode has to fix up the size of the last block
> of the file that was being written to. The namenode contacts the datanodes,
> retrieves the block generation stamp and the length of the lastblock of all
> known replicas, determines which replicas are good and which one are to be
> deleted, stamps the winning replicas with a new generation stamp and deletes
> losing replicas. Once this process is complete, the namenode can allow a new
> writer to append to this file. Details of this design are in HADOOP-1700.
> A few options available to us:
> 1. The namenode sends the request as a response to the next heartbeat RPC
> from the datanode. This methodology is currently used for requesting block
> reports from datanodes.
> 2. The datanode has an RPC server. A pool of threads in the namenode can be
> used to make RPCs to the datanodes.
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