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From "Enis Soztutar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-16894) Create more than 1 split per region, generalize HBASE-12590
Date Sat, 19 Nov 2016 01:33:58 GMT

    [ https://issues.apache.org/jira/browse/HBASE-16894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15678327#comment-15678327
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Enis Soztutar commented on HBASE-16894:
---------------------------------------

bq. We have been discussing an interesting edge case when a table has some skew due the key
design and it would be helpful to write in a single reducer HFiles for multiple regions.
For HBase TableOutputFormat where the output record writer directly writes to HBase, it might
be fine to combine multiple regions in a single reducer since the client will do the correct
thing of finding relevant locations to write to. However, for bulk load and HFileOutputFormat,
we should not have more than 1 region per reduce task. It is "by design" that we should have
at least one reducer per region since the bulk load process cannot load partial ranges from
an hfile. Created HFile must fit into the region boundaries otherwise the BL client will manually
split and rewrite the file, thus causing serial and unnecessary work. 

> Create more than 1 split per region, generalize HBASE-12590
> -----------------------------------------------------------
>
>                 Key: HBASE-16894
>                 URL: https://issues.apache.org/jira/browse/HBASE-16894
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Enis Soztutar
>            Assignee: Yi Liang
>              Labels: beginner, beginners
>
> A common request from users is to be able to better control how many map tasks are created
per region. Right now, it is always 1 region = 1 input split = 1 map task. Same goes for Spark
since it uses the TIF. With region sizes as large as 50 GBs, it is desirable to be able to
create more than 1 split per region.
> HBASE-12590 adds a config property for MR jobs to be able to handle skew in region sizes.
The algorithm is roughly: 
> {code}
> If (region size >= average size*ratio) : cut the region into two MR input splits
> If (average size <= region size < average size*ratio) : one region as one MR input
split
> If (sum of several continuous regions size < average size * ratio): combine these
regions into one MR input split.
> {code}
> Although we can set data skew ratio to be 0.5 or something to abuse HBASE-12590 into
creating more than 1 split task per region, it is not ideal. But there is no way to create
more with the patch as it is. For example we cannot create more than 2 tasks per region. 
> If we want to fix this properly, we should extend the approach in HBASE-12590, and make
it so that the client can specify the desired num of mappers, or desired split size, and the
TIF generates the splits based on the current region sizes very similar to the algorithm in
HBASE-12590, but a more generic way. This also would eliminate the hand tuning of data skew
ratio.
> We also can think about the guidepost approach that Phoenix has in the stats table which
is used for exactly this purpose. Right now, the region can be split into powers of two assuming
uniform distribution within the region. 



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