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Hadoop, mail # user - Re: HDFS short-circuit reads


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Chris Nauroth 2013-12-17, 01:07
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RE: HDFS short-circuit reads
John Lilley 2013-12-17, 14:39
Thanks!   I do call FileSytem.getFileBlockLocations() now to map tasks to local data blocks; is there any advantage to using listLocatedStatus() instead?  I guess one call instead of two...
John
From: Chris Nauroth [mailto:[EMAIL PROTECTED]]
Sent: Monday, December 16, 2013 6:07 PM
To: [EMAIL PROTECTED]
Subject: Re: HDFS short-circuit reads

Hello John,

Short-circuit reads are not on by default.  The documentation page you linked to at hadoop.apache.org<http://hadoop.apache.org/> contains all of the information you need to enable them though.

Regarding checking status of short-circuit read programmatically, here are a few thoughts on this:

Your application could check Configuration for the dfs.client.read.shortcircuit key.  This will tell you at a high level if the feature is enabled.  However, note that the feature needs to be turned on in configuration for both the DataNode and the HDFS client process.  Depending on the details of the deployment, the DataNode and the client might be using different configuration files.

This tells you if the feature is enabled, but it doesn't necessarily tell you if you're really going to get short-circuit reads when you open the file.  There might not be a local replica for the block, in which case the read would fall back to the typical remote read behavior anyway.

Depending on what your application wants to achieve, you might also be interested in looking at the FileSystem.listLocatedStatus API to query information about blocks and the corresponding locations of replicas.  Applications like MapReduce use this information to try to schedule their work for optimal locality.  Short-circuit reads then become a further optimization on top of the gains already achieved by locality.

Hope this helps,

Chris Nauroth
Hortonworks
http://hortonworks.com/
On Mon, Dec 16, 2013 at 4:21 PM, John Lilley <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>> wrote:
Our YARN application would benefit from maximal bandwidth on HDFS reads.
But I'm unclear on how short-circuit reads are enabled.
Are they on by default?
Can our application check programmatically to see if the short-circuit read is enabled?
Thanks,
john

RE:
https://hadoop.apache.org/docs/current/hadoop-project-dist/hadoop-hdfs/ShortCircuitLocalReads.html
https://issues.apache.org/jira/browse/HDFS-347

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John Lilley 2013-12-19, 14:34