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From "Enis Soztutar (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-16894) Create more than 1 split per region, generalize HBASE-12590
Date Thu, 20 Oct 2016 19:06:58 GMT
Enis Soztutar created HBASE-16894:

             Summary: 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

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: 
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.

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

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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