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From Prasanth Jayachandran <pjayachand...@hortonworks.com>
Subject Re: What are all the factors that go into the number of mappers - ORC
Date Mon, 03 Feb 2014 06:22:45 GMT
Hi John

Number of mappers is equal to the number of splits generated. Following are the factors that
go into split generation
1) HDFS block size
2) Max split size

a split is cut when
1) the cumulative size of all adjacent stripes are greater than HDFS block size
2) the cumulative size of all adjacent stripes are greater than max split size

HDFS block size for ORC files will be min(1.5GB, 2*stripe_size) in the current version of
hive (and probably hive 0.12 too). In older versions, HDFS block size = min(2GB, 2*stripe_size).


The other important thing to note is ORC split is generated only when HiveInputFormat is used.
By default hive uses CombineHiveInputFormat which uses a different strategy to generate splits.
In CombineHiveInputFormat, many small files are combined together to form a large logical
split.

In any case for the size you had mentioned (2000 bytes) there should be only one mapper. Can
you provide the value for following configs so that we can understand it better?

1) hive.input.format
2) hive.min.split.size
3) hive.max.split.size
4) total size on disk for the table

Thanks
Prasanth Jayachandran

On Feb 2, 2014, at 5:25 PM, John Omernik <john@omernik.com> wrote:

> I have two clusters, but small dev clusters, and I loaded the same dataset into both
of them.   The data size on disk is within 2000 Bytes. Both are ORC, one is Hive 11 and one
is Hive 12.  One is allocating about 8 more mappers to the exact same query. I am just curious
what settings would change that. I checked through all my setting, but can't see what would
cause the discrepancy. Is this an ORC v11 vs v12 thing?
> 
> I'd be curious on the thoughts of the group.


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