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https://issues.apache.org/jira/browse/HBASE-16894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15688128#comment-15688128
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Dave Latham commented on HBASE-16894:
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Deriving split points from the actual distribution of data as captured in
HFiles sounds great. However, as a user of a large cluster I'm worried that a
single threaded process doing HDFS operations for every region in a table with
10k, 100k, or more regions could take a great deal of time and/or bottleneck on
the NameNode. Since the region servers already likely have the index data
cached, taking advantage of it would be great if possible.
> 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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