[jira] [Commented] (HADOOP-12878) Impersonate hosts in s3a for better data locality handling

["Steve Loughran (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-12878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15282631#comment-15282631 ] 

Steve Loughran commented on HADOOP-12878:
-----------------------------------------

Link to the azure code here: https://github.com/apache/hadoop/blob/trunk/hadoop-tools/hadoop-azure/src/main/java/org/apache/hadoop/fs/azure/NativeAzureFileSystem.java#L2550-L2589

> 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
>            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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