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From "Thomas Demoor (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-12878) Impersonate hosts in s3a for better data locality handling
Date Fri, 10 Jun 2016 11:30:21 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-12878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15324313#comment-15324313

Thomas Demoor commented on HADOOP-12878:

[~cnauroth]: I assumed this would be used as follows: by having  {{fs.wasb/s3a.block.location.impersonatedhost=host1,host2,...,host50}}
 you would impersonate the block to be local on all nodes, the scheduler would then be offered
options. An optimization would be to return 5 random hosts.
[~dweeks]: Thanks for your interest!
* The current s3a implementation causes scheduling delays as it declares localhost as the
only locality for all blocks. 
* The proposed solution can indeed be suboptimal under load but it's always an improvement
over the current situation
* I agree that using "*"  woud be easier and more correct but I think [~stevel@apache.org]
might have a point. Testing... 
* Returning the s3 endpoint so things are always off-rack seems like an interesting idea.

[~PieterReuse] has verified that what I originally proposed speeds things up. We are checking
if any of Daniel's proposals would work out as well. Expect an update next week.

> Impersonate hosts in s3a for better data locality handling
> ----------------------------------------------------------
>                 Key: HADOOP-12878
>                 URL: https://issues.apache.org/jira/browse/HADOOP-12878
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 2.8.0
>            Reporter: Thomas Demoor
>            Assignee: Thomas Demoor
> Currently, {{localhost}} is passed as locality for each block, causing all blocks involved
in job to initially target the same node (RM), before being moved by the scheduler (to a rack-local
node). This reduces parallelism for jobs (with short-lived mappers). 
> We should mimic Azures implementation: a config setting {{fs.s3a.block.location.impersonatedhost}}
where the user can enter the list of hostnames in the cluster to return to {{getFileBlockLocations}}.

> Possible optimization: for larger systems, it might be better to return N (5?) random
hostnames to prevent passing a huge array (the downstream code assumes size = O(3)).

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