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Posted to mapreduce-issues@hadoop.apache.org by "Steve Loughran (Jira)" <ji...@apache.org> on 2020/09/14 21:54:00 UTC
[jira] [Commented] (MAPREDUCE-7282) MR v2 commit algorithm is
dangerous, should be deprecated and not the default
[ https://issues.apache.org/jira/browse/MAPREDUCE-7282?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17195750#comment-17195750 ]
Steve Loughran commented on MAPREDUCE-7282:
-------------------------------------------
Further reading: https://github.com/steveloughran/zero-rename-committer/releases/tag/tag_draft_006
> MR v2 commit algorithm is dangerous, should be deprecated and not the default
> -----------------------------------------------------------------------------
>
> Key: MAPREDUCE-7282
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-7282
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: mrv2
> Affects Versions: 3.3.0, 3.2.1, 3.1.3, 3.3.1
> Reporter: Steve Loughran
> Priority: Major
>
> The v2 MR commit algorithm moves files from the task attempt dir into the dest dir on task commit -one by one
> It is therefore not atomic
> # if a task commit fails partway through and another task attempt commits -unless exactly the same filenames are used, output of the first attempt may be included in the final result
> # if a worker partitions partway through task commit, and then continues after another attempt has committed, it may partially overwrite the output -even when the filenames are the same
> Both MR and spark assume that task commits are atomic. Either they need to consider that this is not the case, we add a way to probe for a committer supporting atomic task commit, and the engines both add handling for task commit failures (probably fail job)
> Better: we remove this as the default, maybe also warn when it is being used
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