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From Arpit Gupta <ar...@hortonworks.com>
Subject Re: why my test result on dfs short circuit read is slower?
Date Sat, 16 Feb 2013 06:05:57 GMT
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.

On Feb 15, 2013, at 9:53 PM, "Liu, Raymond" <raymond.liu@intel.com> 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 <raymond.liu@intel.com>
>> 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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