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https://issues.apache.org/jira/browse/HDFS-1143?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12868384#action_12868384
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Scott Chen commented on HDFS-1143:
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Yes, that's what we meant, Koji.
Actually the patch will also improve the namenode response (for other clients) 
because we move the collectSubtreeBlocksAndClear() in the background.
This method walks the detached subtree and collects blocks. It is currently 
running under the global lock.

> Implement Background deletion
> -----------------------------
>
>                 Key: HDFS-1143
>                 URL: https://issues.apache.org/jira/browse/HDFS-1143
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: name-node
>    Affects Versions: 0.22.0
>            Reporter: Dmytro Molkov
>            Assignee: Scott Chen
>             Fix For: 0.22.0
>
>         Attachments: HDFS-1143.txt
>
>
> Right now if you try to delete massive number of files from the namenode it 
> will freeze (sometimes for minutes). Most of the time is spent going through 
> the blocks map and invalidating all the blocks.
> This can probably be improved by having a background GC process. The deletion 
> will basically just remove the inode being deleted and then give the subtree 
> that was just deleted to the background thread running cleanup.
> This way the namenode becomes available for the clients soon after deletion, 
> and all the heavy operations are done in the background.
> Thoughts?

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