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https://issues.apache.org/jira/browse/HADOOP-3201?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12590252#action_12590252
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dhruba borthakur commented on HADOOP-3201:
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1. The namenode asks the primary datanode (through the heartbeat response
mechanism) to do a lease recovery. It specifies the blockid in the heartbeat
response.
2. The primary datanode receives the blocks for which a lease recovery is
needed. It contacts the specified datanode(s) in the pipeline, retrieves
blockids and genstamps from them. It then makes a RPC to the namenode
specifying the generation stamps of the replicas. The namenode then decides
which replicas to keep and which to delete and make necessary midifications to
the blocksMap. It also removes the lease associated with the file, thereby
indicating the lease recovery is complete.
3. Each datanode has an RPC server. The primary datanode makes an RPC to the
secondary datanode(s) in the pipeline to retrieve generation stamp of a
block(s).
> 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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