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https://issues.apache.org/jira/browse/HIVE-1118?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12806435#action_12806435
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Zheng Shao commented on HIVE-1118:
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That's still much better than NOT running the merge task.
With the currently setting, almost nobody will enable this by default. As a
result, we are seeing a lot of 1KB files in the HDFS.
If we make this change, we can enable it by default.
I agree 1MB is not a good default. We can set it to 32MB or 64MB (and on by
default).
If that's not good enough, let's introduce another parameter so we can say
(32MB, 64MB), which will start the merge job if average size of file is smaller
than 32MB, and we will end up with files with 64MB.
Thoughts?
> Hive merge map files should have different bytes/mapper setting
> ---------------------------------------------------------------
>
> Key: HIVE-1118
> URL: https://issues.apache.org/jira/browse/HIVE-1118
> Project: Hadoop Hive
> Issue Type: Improvement
> Reporter: Zheng Shao
>
> Currently, by default, we get one reducer for each 1GB of input data.
> It's also true for the conditional merge job that will run if the average
> file size is smaller than a threshold.
> This actually makes those job very slow, because each reducer needs to
> consume 1GB of data.
> Alternatively, we can just use that threshold to determine the number of
> reducers per job (or introduce a new parameter).
> Let's say the threshold is 1MB, then we only start the the merge job if the
> average file size is less than 1MB, and the eventual result file size will be
> around 1MB (or another small number).
> This will remove the extreme cases where we have thousands of empty files,
> but still make normal jobs fast enough.
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