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From Igor Tatarinov <>
Subject Re: question about number of map tasks for small file
Date Wed, 01 Jun 2011 17:12:06 GMT
Can you pre-aggregate your historical data to reduce the number of files?

We used to partition our data by date but that created too many output files
so now we partition by month.

I do find it odd that Hive (0.6) can't merge compressed output files. We
could have gotten away with daily partitioning if Hive could merge small
files. I tried disabling compression but it actually caused some execution
problems (perhaps xcievers -related I am not sure)

On Wed, Jun 1, 2011 at 12:38 AM, Junxian Yan <> wrote:

> Today I tried CombineHiveInputFormat and set the max split size for hadoop
> input. Seems I can get the expected map tasks number. But another problem is
> the cpu is consumed highly by map tasks. almost 100%.
> I just ran a query with simple WHERE condition over testing files,whose
> total size is about 30M and there are about 10 thousand small files. The
> execution time over 700s. It's killing us.  Because the files are generated
> by flume, all files is seq file.
> R
> On Tue, May 31, 2011 at 2:55 AM, Junxian Yan <>wrote:
>> Hi Guys
>> I use flume to store log file , and use hive to query.
>> Flume always store the small file with suffix .seq Now I have over 35
>> thousand seq files. Every time when I launch query script, 35 thousand map
>> tasks will be created and it's so long time to wait for completing.
>> I also try to set CombineHiveInputFormat, but if I set this option, it
>> seems the task will be executed slowly. Because total size of the data
>> folder over 700M.  Now in my testing env, I only have 3 data nodes. I also
>> tried to add after the CombineHiveInputFormat setting,
>> seems doesn't work. There's alway only one map task if
>> set CombineHiveInputFormat.
>> Can you plz show me a solution in which I can set map task number freely
>> BTW: version for hadoop is 20 and hive is 0.5
>> Richard

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