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https://issues.apache.org/jira/browse/HBASE-7743?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13797120#comment-13797120
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Nick Dimiduk commented on HBASE-7743:
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bq. Having reducers group all cells per row will not handle very large rows
very well.
That's precisely the reason I stumbled into this in the first place :)
> Replace *SortReducers with Hadoop Secondary Sort
> ------------------------------------------------
>
> Key: HBASE-7743
> URL: https://issues.apache.org/jira/browse/HBASE-7743
> Project: HBase
> Issue Type: Sub-task
> Components: mapreduce, Performance
> Reporter: Nick Dimiduk
>
> The mapreduce package provides two Reducer implementations,
> KeyValueSortReducer and PutSortReducer, which are used by Import, ImportTsv,
> and WALPlayer in conjunction with the HFileOutputFormat. Both of these
> implementations make use of a TreeSet to sort values matching a key. This
> reducer will OOM when rows are large.
> A better solution would be to implement secondary sort of the values. That
> way hadoop sorts the records, spilling to disk when necessary.
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