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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3201) namenode should be able to retrieve block metadata from a datanode
Date Fri, 18 Apr 2008 00:11:23 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3201?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12590252#action_12590252
] 

dhruba borthakur commented on HADOOP-3201:
------------------------------------------

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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