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From John Lilley <john.lil...@redpoint.net>
Subject RE: HDFS short-circuit reads
Date Tue, 17 Dec 2013 14:39:01 GMT
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

From: Chris Nauroth [mailto:cnauroth@hortonworks.com]
Sent: Monday, December 16, 2013 6:07 PM
To: user@hadoop.apache.org
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

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

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

On Mon, Dec 16, 2013 at 4:21 PM, John Lilley <john.lilley@redpoint.net<mailto:john.lilley@redpoint.net>>
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?


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