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https://issues.apache.org/jira/browse/HDFS-854?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12803501#action_12803501
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Lohit Vijayarenu commented on HDFS-854:
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Or probably keep last added/deleted blocks in memory and send block report and 
doing disk scan once in a while?

> Datanode should scan devices in parallel to generate block report
> -----------------------------------------------------------------
>
>                 Key: HDFS-854
>                 URL: https://issues.apache.org/jira/browse/HDFS-854
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: data-node
>            Reporter: dhruba borthakur
>            Assignee: Dmytro Molkov
>
> A Datanode should scan its disk devices in parallel so that the time to 
> generate a block report is reduced. This will reduce the startup time of a 
> cluster.
> A datanode has 12 disk (each of 1 TB) to store HDFS blocks. There is a total 
> of 150K blocks on these 12 disks. It takes the datanode upto 20 minutes to 
> scan these devices to generate the first block report.

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