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Hive >> mail # user >> What are all the factors that go into the number of mappers - ORC


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Re: What are all the factors that go into the number of mappers - ORC
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 <[EMAIL PROTECTED]> 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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