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MapReduce >> mail # user >> Re: why my test result on dfs short circuit read is slower?


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Harsh J 2013-02-16, 05:44
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Liu, Raymond 2013-02-16, 05:53
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Re: why my test result on dfs short circuit read is slower?
Another way to check if short circuit read is configured correctly.

As the user who is configured for short circuit read issue the following command on a node where you expect the data to be read locally.

export HADOOP_ROOT_LOGGER=debug,console; hadoop dfs -cat /path/to/file_on_hdfs

On the console you should see something like "hdfs.DFSClient: New BlockReaderLocal for file...."

This would confirm that short circuit read is happening.

--
Arpit Gupta
Hortonworks Inc.
http://hortonworks.com/

On Feb 15, 2013, at 9:53 PM, "Liu, Raymond" <[EMAIL PROTECTED]> wrote:

> Hi Harsh
>
> Yes, I did set both of these. While not in hbase-site.xml but hdfs-site.xml.
>
> And I have double confirmed that local reads are performed, since there are no Error in datanode logs, and by watching lo network IO.
>
>>
>> If you want HBase to leverage the shortcircuit, the DN config
>> "dfs.block.local-path-access.user" should be set to the user running HBase (i.e.
>> hbase, for example), and the hbase-site.xml should have
>> "dfs.client.read.shortcircuit" defined in all its RegionServers. Doing this wrong
>> could result in performance penalty and some warn-logging, as local reads will
>> be attempted but will begin to fail.
>>
>> On Sat, Feb 16, 2013 at 8:40 AM, Liu, Raymond <[EMAIL PROTECTED]>
>> wrote:
>>> Hi
>>>
>>>        I tried to use short circuit read to improve my hbase cluster MR
>> scan performance.
>>>
>>>        I have the following setting in hdfs-site.xml
>>>
>>>        dfs.client.read.shortcircuit set to true
>>>        dfs.block.local-path-access.user set to MR job runner.
>>>
>>>        The cluster is 1+4 node and each data node have 16cpu/4HDD,
>> with all hbase table major compact thus all data is local.
>>>        I have hoped that the short circuit read will improve the
>> performance.
>>>
>>>        While the test result is that with short circuit read enabled, the
>> performance actually dropped 10-15%. Say scan a 50G table cost around 100s
>> instead of 90s.
>>>
>>>        My hadoop version is 1.1.1, any idea on this? Thx!
>>>
>>> Best Regards,
>>> Raymond Liu
>>>
>>>
>>
>>
>>
>> --
>> Harsh J

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Liu, Raymond 2013-02-16, 06:22
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Liu, Raymond 2013-02-16, 07:54
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谢良 2013-02-16, 04:04