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From Hadi Moshayedi <h...@moshayedi.net>
Subject Re: Compression of Intermediate Data
Date Mon, 08 Oct 2012 15:17:41 GMT
Hi Bejoy,
  Thanks.

  Following your instructions, I also enabled map output compression.

  I tried different queries but I couldn't get the benefit from compression
in any single of them. I also tried creating queries which have large
intermediate data, but it didn't improve the performance for them either.

  I should also note that our Hadoop cluster is setup at few Amazon EC2
m2.2xlarge instances.

  Question is: What are the scenarios in which compression can improve the
performance?


 Thanks,
   -- Hadi

On Sat, Oct 6, 2012 at 6:32 PM, Bejoy KS <bejoy_ks@yahoo.com> wrote:

> **
> Hi Hadi
>
> The propertis you specified doen't enable compression of map output. To
> enable map output compression you need to enable the following properties
>
> SET hive.exec.compress.output=true;
> ‪
> SET mapred.map.output.compression=true;
> SET
> mapred.map.output.compression.codec=org.apache.hadoop.io.compress.SnappyCodec;
>
>
> This property 'hive.exec.compress.intermediate
> ' Is used to enable compression of data in between multiple mapreduce jobs
> generated by a hive query.
> Regards
> Bejoy KS
>
> Sent from handheld, please excuse typos.
> ------------------------------
> *From: * Hadi Moshayedi <hadi@moshayedi.net>
> *Date: *Sat, 6 Oct 2012 16:55:47 +0300
> *To: *<user@hive.apache.org>
> *ReplyTo: * user@hive.apache.org
> *Subject: *Compression of Intermediate Data
>
> I wanted to look into improving performance of my Hive cluster, and from
> what I read turning on compression of intermediate data could help. As I
> understand, this would help because it would reduce the amount of data
> written to disk in between jobs.
>
> I look at the documentation and set the following settings:
>
> SET hive.exec.compress.intermediate=true;
> SET mapred.output.compression.type=BLOCK;
> SET
> mapred.map.output.compression.codec=org.apache.hadoop.io.compress.SnappyCodec;
>
>  I ran some queries to see how compression impacts the performance. But it
> usually made the query time worse. I also had a query whose size of
> intermediate data was close to the size of input data, but it made the
> performance worse for this query too.
>
>  Question 1: Are the above settings correct settings for using compression
> of intermediate data?
>
>  Question 2: Are there use-cases in which compression of intermediate data
> would not help performance? Why would someone not keep it turned on always?
>
> Thanks
>

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