FAQ
I don't understand the internal logic of the FileSplit and Mapper.

By my understanding, I think FileInputFormat is the actual class that
takes care of the file spliting. So it's reasonable if one large file is
splited into 5 smaller parts, each parts is less than 2GB (since we
specify the numberOfSplit is 5).

However, the FileSplit is rough edges, so mapper 1 which takes the split
1 as input omit the incomplete parts at the end of split 1, then mapper
2 will continue to read that incomplete part then add the remaining part
of split 2?

Take this as example:

The original file is:

1::122::5::838985046 (CRLF)
1::185::5::838983525 (CRLF)
1::231::5::838983392 (CRLF)

Assume number of split is 2, then the above content is divied into two
part:

Split 1:
1::122::5::838985046 (CRLF)
1::185::5::8


Split 2:
38983525 (CRLF)
1::231::5::838983392 (CRLF)

Afterwards, Mapper 1 takes split 1 as input, but after eat the line
1::122::5::838985046, it found the remaining part is not a complete
record, then Mapper 1 bypass it, but Mapper 2 will read this and add it
ahead of first line of Split 2 to compose a valid record.

Is it correct ? If it is, which class implements the above logic?

BR/anderson

-----Original Message-----
From: Aaron Kimball
Sent: Thursday, June 11, 2009 11:49 AM
To: core-user@hadoop.apache.org
Subject: Re: Large size Text file split

The FileSplit boundaries are "rough" edges -- the mapper responsible for
the previous split will continue until it finds a full record, and the
next mapper will read ahead and only start on the first record boundary
after the byte offset.
- Aaron
On Wed, Jun 10, 2009 at 7:53 PM, Wenrui Guo wrote:

I think the default TextInputFormat can meet my requirement. However,
even if the JavaDoc of TextInputFormat says the TextInputFormat could
divide input file as text lines which ends with CRLF. But I'd like to
know if the FileSplit size is not N times of line length, what will be
happen eventually?

BR/anderson

-----Original Message-----
From: jason hadoop
Sent: Wednesday, June 10, 2009 8:39 PM
To: core-user@hadoop.apache.org
Subject: Re: Large size Text file split

There is always NLineInputFormat. You specify the number of lines per
split.
The key is the position of the line start in the file, value is the
line itself.
The parameter mapred.line.input.format.linespermap controls the number
of lines per split

On Wed, Jun 10, 2009 at 5:27 AM, Harish Mallipeddi <
harish.mallipeddi@gmail.com> wrote:
On Wed, Jun 10, 2009 at 5:36 PM, Wenrui Guo
<wenrui.guo@ericsson.com>
wrote:
Hi, all

I have a large csv file ( larger than 10 GB ), I'd like to use a
certain InputFormat to split it into smaller part thus each Mapper
can deal with piece of the csv file. However, as far as I know,
FileInputFormat only cares about byte size of file, that is, the
class can divide the csv file as many part, and maybe some part is
not a well-format CVS file.
For example, one line of the CSV file is not terminated with CRLF,
or maybe some text is trimed.

How to ensure each FileSplit is a smaller valid CSV file using a
proper InputFormat?

BR/anderson
If all you care about is the splits occurring at line boundaries,
then
TextInputFormat will work.

http://hadoop.apache.org/core/docs/current/api/org/apache/hadoop/map
re
d/TextInputFormat.html

If not I guess you can write your own InputFormat class.

--
Harish Mallipeddi
http://blog.poundbang.in


--
Pro Hadoop, a book to guide you from beginner to hadoop mastery,
http://www.apress.com/book/view/9781430219422
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