[jira] [Commented] (HDFS-5446) Consider supporting a mechanism to allow datanodes to drain outstanding work during rolling upgrade

["Andrew Wang (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HDFS-5446?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13810911#comment-13810911 ] 

Andrew Wang commented on HDFS-5446:
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Very interesting idea, thanks for filing this Nathan. Are you thinking this would be a NN-side thing like DN decommissioning? The two are kind of similar; decommissioning DNs aren't assigned more blocks to write, and I believe are deprioritized for reads as well. For rolling restart, we just wouldn't be moving the blocks off. It'd be up to the admin to toggle/untoggle this state as the rolling restart progresses.

> Consider supporting a mechanism to allow datanodes to drain outstanding work during rolling upgrade
> ---------------------------------------------------------------------------------------------------
>
>                 Key: HDFS-5446
>                 URL: https://issues.apache.org/jira/browse/HDFS-5446
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: datanode
>    Affects Versions: 2.2.0
>            Reporter: Nathan Roberts
>
> Rebuilding write pipelines is expensive and this can happen many times during a rolling restart of datanodes (i.e. during a rolling upgrade). It seems like it might help if datanodes could be told to drain current work while rejecting new requests - possibly with a new response indicating the node is temporarily unavailable (it's not broken, it's just going through a maintenance phase where it shouldn't accept new work). 
> Waiting just a few seconds is normally enough to clear up a good percentage of the open requests without error, thus reducing the overhead associated with restarting lots of datanodes in rapid succession.
> Obviously would need a timeout to make sure the datanode doesn't wait forever.



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