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From "Liu, Raymond" <raymond....@intel.com>
Subject RE: why my test result on dfs short circuit read is slower?
Date Sat, 16 Feb 2013 06:22:39 GMT
Hi Arpit Gupta

Yes,  this way also confirms that short circuit read is enabled on my cluster.

13/02/16 14:07:34 DEBUG hdfs.DFSClient: Short circuit read is true

13/02/16 14:07:34 DEBUG hdfs.DFSClient: New BlockReaderLocal for file /mnt/DP_disk4/raymond/hdfs/data/current/subdir63/blk_-2736548898990727638
of size 134217728 startOffset 0 length 134217728 short circuit checksum false

So , any possibility that other setting might impact short circuit read to has worse performance
than read through datanode?


>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

>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>


       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

       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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