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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:
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> 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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