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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1295) Improve namenode restart times by short-circuiting the first block reports from datanodes
Date Thu, 16 Sep 2010 16:45:40 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1295?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12910218#action_12910218
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dhruba borthakur commented on HDFS-1295:
----------------------------------------

> because the actual speedup is achieved because you reuse the same Block object

I did an experiment to not use the same Block object with the new shortcircuiting code, the
performance gain remains the same. 

> The trade-off is: you will start faster, but space cleanup will be delayed.
> That sounds like a fine tradeoff, don't you agree? 
 
Thanks luoli for your comments. Konstantin: do you agree that this tradeoff looks fine to
you?

> Improve namenode restart times by short-circuiting the first block reports from datanodes
> -----------------------------------------------------------------------------------------
>
>                 Key: HDFS-1295
>                 URL: https://issues.apache.org/jira/browse/HDFS-1295
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>             Fix For: 0.22.0
>
>         Attachments: shortCircuitBlockReport_1.txt
>
>
> The namenode restart is dominated by the performance of processing block reports. On
a 2000 node cluster with 90 million blocks,  block report processing takes 30 to 40 minutes.
The namenode "diffs" the contents of the incoming block report with the contents of the blocks
map, and then applies these diffs to the blocksMap, but in reality there is no need to compute
the "diff" because this is the first block report from the datanode.
> This code change improves block report processing time by 300%.

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