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From Abraham Elmahrek <...@cloudera.com>
Subject Re: Strange distribution of keys among mappers
Date Wed, 19 Jun 2013 23:50:39 GMT
David,

Well I think sqoop is looking at "mapred.map.tasks". Do you have that set
in mapred-site.xml? I thought that defaults to 2.

-Abe


On Wed, Jun 19, 2013 at 4:31 PM, Abraham Elmahrek <abe@cloudera.com> wrote:

> David,
>
> I've created https://issues.apache.org/jira/browse/SQOOP-1093 to track
> the documentation issue. Thanks for bringing this to the community's
> attention!
>
> -Abe
>
>
> On Wed, Jun 19, 2013 at 4:21 PM, Abraham Elmahrek <abe@cloudera.com>wrote:
>
>> Hey David,
>>
>> With oracle, the BigDecimalSplitter will be used by default for all
>> number types.
>>
>> -Abe
>>
>>
>> On Wed, Jun 19, 2013 at 4:05 PM, David Kincaid <kincaid.dave@gmail.com>wrote:
>>
>>> Abe, the database is Oracle.
>>>
>>>
>>> On Wed, Jun 19, 2013 at 5:48 PM, Abraham Elmahrek <abe@cloudera.com>wrote:
>>>
>>>> David,
>>>>
>>>> What database are you importing from? The description I gave was for
>>>> datatypes that map to the BigDecimal Splitter. The userguide might be
>>>> referring to the IntegerSplitter which will add the remainder to the last
>>>> value.
>>>>
>>>> -Abe
>>>>
>>>>
>>>> On Wed, Jun 19, 2013 at 1:23 PM, David Kincaid <kincaid.dave@gmail.com>wrote:
>>>>
>>>>> Thanks. We didn't specify the number of mappers, so it's giving us 4.
>>>>> I understand your explanation, but it seems to conflict with the Sqoop
user
>>>>> guide (
>>>>> http://sqoop.apache.org/docs/1.4.3/SqoopUserGuide.html#_controlling_parallelism
>>>>> ):
>>>>>
>>>>> "When performing parallel imports, Sqoop needs a criterion by which
>>>>> it can split the workload. Sqoop uses a *splitting column* to split
>>>>> the workload. By default, Sqoop will identify the primary key column
(if
>>>>> present) in a table and use it as the splitting column. The low and high
>>>>> values for the splitting column are retrieved from the database, and
the
>>>>> map tasks operate on evenly-sized components of the total range. For
>>>>> example, if you had a table with a primary key column of id whose
>>>>> minimum value was 0 and maximum value was 1000, and Sqoop was directed
to
>>>>> use 4 tasks, Sqoop would run four processes which each execute SQL
>>>>> statements of the form SELECT * FROM sometable WHERE id >= lo AND
id
>>>>> < hi, with (lo, hi) set to (0, 250), (250, 500), (500, 750), and
>>>>> (750, 1001) in the different tasks."
>>>>>
>>>>>
>>>>> On Wed, Jun 19, 2013 at 3:14 PM, Abraham Elmahrek <abe@cloudera.com>wrote:
>>>>>
>>>>>> Hey David,
>>>>>>
>>>>>> Here's the algorithm:
>>>>>> Split lengths are defined by (max - min)/(# mappers) and whatever
is
>>>>>> left is tacked on at the end. So in this case, (288272191-2110)/3
=
>>>>>> 96090027.33... So I'm assuming the .33... is rounded down and split
lengths
>>>>>> will be of length 96090027. Sqoop will then create splits with the
>>>>>> following points: (min) + (range length)*(n). We can see that 2110
+ 96090027*0
>>>>>> = 2110, 2110 + 96090027*1 = 96092137, 2110 + 96090027*2 = 192182164,
>>>>>> and 2110 + 96090027*3 = 288272191 will be generated based off of
>>>>>> this algorithm. The last point to be added will be 288272192 because
>>>>>> the max value is not part of the generated split points. Then sqoop
will
>>>>>> distributed accordingly based off of these points as you've pointed
out
>>>>>> above.
>>>>>>
>>>>>> Just to be sure, did you configure sqoop to use 3 mappers?
>>>>>>
>>>>>> Hope this helps,
>>>>>> -Abe
>>>>>>
>>>>>>
>>>>>> On Wed, Jun 19, 2013 at 8:33 AM, David Kincaid <
>>>>>> kincaid.dave@gmail.com> wrote:
>>>>>>
>>>>>>> We're seeing a strange thing happen with a sqoop import job with
the
>>>>>>> way the key range is getting distributed among the 4 mappers
that are
>>>>>>> running. The minimum key value is 2110 and the maximum value
is 288272191.
>>>>>>> We are getting one mapper that is only getting one record to
import. Here
>>>>>>> is the distribution among the mappers:
>>>>>>>
>>>>>>> [2110, 96092137)
>>>>>>> [96092137, 192182164)
>>>>>>> [192182164, 288272191)
>>>>>>> [288272191, 288272192)
>>>>>>>
>>>>>>> you can see that the fourth mapper is given a range with only
one
>>>>>>> value in it. Could someone help me understand what is going on?
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> Dave
>>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

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