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Date: Mon, 22 Aug 2011 22:20:29 +0000 (UTC)
From: "Sanjay Radia (JIRA)" <jira@apache.org>
To: hdfs-issues@hadoop.apache.org
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Subject: [jira] [Commented] (HDFS-1971) HA: Send block report from datanode
 to both active and standby namenodes
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    [ https://issues.apache.org/jira/browse/HDFS-1971?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13089075#comment-13089075 ] 

Sanjay Radia commented on HDFS-1971:
------------------------------------

@Uma descries a situation where the standby sends a command to DN when it receives a block info for a file under-construction before reading the the editsLog from the active NN for that file.

The design posted in HDFS-1623 requires that the DN accepts commands only from the Active. Further the NN in standby state will not issue commands to any DN. 

However your scenario is a good one: a standby processing BRs from DNs and edits from Active NN may get them out of order.
This scenario applies to HDFS-9175 that reads from a shared editsLog file system  and also for Backup NN.


> HA: Send block report from datanode to both active and standby namenodes
> ------------------------------------------------------------------------
>
>                 Key: HDFS-1971
>                 URL: https://issues.apache.org/jira/browse/HDFS-1971
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: data-node, name-node
>            Reporter: Suresh Srinivas
>            Assignee: Sanjay Radia
>         Attachments: DualBlockReports.pdf
>
>
> To enable hot standby namenode, the standby node must have current information for - namenode state (image + edits) and block location information. This jira addresses keeping the block location information current in the standby node. To do this, the proposed solution is to send block reports from the datanodes to both the active and the standby namenode.

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