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From Gang Luo <lgpub...@yahoo.com.cn>
Subject Re: map side only behavior
Date Fri, 29 Jan 2010 16:30:19 GMT
Thanks Jeff.

But what if the mapper's output size surpass the capacity of the output buffer? There will
be multiple spills for this mapper in this case. If there is no merging, how to ensure there
is *only one* output file?

 
-Gang



----- 原始邮件 ----
发件人: Jeff Zhang <zjffdu@gmail.com>
收件人: common-user@hadoop.apache.org
发送日期: 2010/1/29 (周五) 11:05:33 上午
主   题: Re: map side only behavior

No, the merge and sort will not happen in mapper task. And each mapper task
will generate one output file.



2010/1/29 Gang Luo <lgpublic@yahoo.com.cn>

> Hi all,
> If I only use map side to process my data (set # of reducers to 0 ), what
> is the behavior of hadoop? Will it merge and sort each of the spills
> generated by one mapper?
>
> -Gang
>
>
> ----- 原始邮件 ----
> 发件人: Gang Luo <lgpublic@yahoo.com.cn>
> 收件人: common-user@hadoop.apache.org
> 发送日期: 2010/1/29 (周五) 8:54:33 上午
> 主   题: Re: fine granularity operation on HDFS
>
> Yeah, I see how it works. Thanks Amogh.
>
>
> -Gang
>
>
>
> ----- 原始邮件 ----
> 发件人: Amogh Vasekar <amogh@yahoo-inc.com>
> 收件人: "common-user@hadoop.apache.org" <common-user@hadoop.apache.org>
> 发送日期: 2010/1/28 (周四) 10:00:22 上午
> 主   题: Re: fine granularity operation on HDFS
>
> Hi Gang,
> Yes PathFilters work only on file paths. I meant you can include such type
> of logic at split level.
> The input format's getSplits() method is responsible for computing and
> adding splits to a list container, for which JT initializes mapper tasks.
> You can override the getSplits() method to add only a few , say, based on
> the location or offset, to the list. Here's the reference :
> while (((double) bytesRemaining)/splitSize > SPLIT_SLOP) {
>          int blkIndex = getBlockIndex(blkLocations, length-bytesRemaining);
>          splits.add(new FileSplit(path, length-bytesRemaining, splitSize,
>                                   blkLocations[blkIndex].getHosts()));
>          bytesRemaining -= splitSize;
>        }
>
>        if (bytesRemaining != 0) {
>          splits.add(new FileSplit(path, length-bytesRemaining,
> bytesRemaining,
>                     blkLocations[blkLocations.length-1].getHosts()));
>
> Before splits.add you can use your logic for discarding. However, you need
> to ensure your record reader takes care of incomplete records at boundaries.
>
> To get the block locations to load separately, the FileSystem class APIs
> expose few methods like getBlockLocations etc ..
> Hope this helps.
>
> Amogh
>
> On 1/28/10 7:26 PM, "Gang Luo" <lgpublic@yahoo.com.cn> wrote:
>
> Thanks Amogh.
>
> For the second part of my question, I actually mean loading block
> separately from HDFS. I don't know whether it is realistic. Anyway, for my
> goal is to process different division of a file separately, to do that at
> split level is OK. But even I can get the splits from inputformat, how to
> "add only a few splits you need to mapper and discard the others"?
> (pathfilters only works for file, but not block, I think).
>
> Thanks.
> -Gang
>
>
>
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-- 
Best Regards

Jeff Zhang



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  好玩贺卡等你发,邮箱贺卡全新上线! 
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