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From "Doug Cutting (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3201) namenode should be able to retrieve block metadata from a datanode
Date Mon, 07 Apr 2008 19:51:24 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3201?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12586512#action_12586512

Doug Cutting commented on HADOOP-3201:

Unless is proves unfeasable, it seems preferable to not add a new RPC protocol, daemon and
namenode thread pool, but rather to transmit this in heartbeat responses.  Lease recovery
does not seem to be performance sensitive.

> 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
> 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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