When you use the HDFS client interface to read a file, it automatically figures out which datanodes to contact for reading which blocks. There isn't really a "main" block. However I have read that the first location listed for each block is the "recommended" one to read for an outside client. Normally, an outside client doesn't need to know this information at all as the HDFS file interface takes care of it. An "inside" application such as MapReduce *does* need to know this information so that it can run tasks on nodes that are "close" to the data split being processed. If you are writing a custom ApplicationMaster using YARN, you will also want to know this.
John From: Mahmood Naderan [mailto:nt_mahm...@yahoo.com] Sent: Tuesday, June 04, 2013 12:01 AM To: user@hadoop.apache.org Subject: Re: HDFS interfaces There are many instances of getFileBlockLocations in hadoop/fs. Can you explain which one is the main? >It must be combined with a method of logically splitting the input data along >block boundaries, and of launching tasks on worker nodes that >are close to >the data splits Is this a user level task of system level task? Regards, Mahmood ________________________________ From: John Lilley <john.lil...@redpoint.net<mailto:john.lil...@redpoint.net>> To: "user@hadoop.apache.org<mailto:user@hadoop.apache.org>" <user@hadoop.apache.org<mailto:user@hadoop.apache.org>>; Mahmood Naderan <nt_mahm...@yahoo.com<mailto:nt_mahm...@yahoo.com>> Sent: Tuesday, June 4, 2013 3:28 AM Subject: RE: HDFS interfaces Mahmood, It is the in the FileSystem interface. http://hadoop.apache.org/docs/current/api/org/apache/hadoop/fs/FileSystem.html#getFileBlockLocations(org.apache.hadoop.fs.Path, long, long)<http://hadoop.apache.org/docs/current/api/org/apache/hadoop/fs/FileSystem.html#getFileBlockLocations(org.apache.hadoop.fs.Path,%20long,%20long)> This by itself is not sufficient for application programmers to make good use of data locality. It must be combined with a method of logically splitting the input data along block boundaries, and of launching tasks on worker nodes that are close to the data splits. MapReduce does both of these things internally along with the file-format input classes. For an application to do so directly, see the new YARN-based interfaces ApplicationMaster and ResourceManager. These are however very new and there is little documentation or examples. john From: Mahmood Naderan [mailto:nt_mahm...@yahoo.com] Sent: Monday, June 03, 2013 12:09 PM To: user@hadoop.apache.org<mailto:user@hadoop.apache.org> Subject: HDFS interfaces Hello, It is stated in the "HDFS architecture guide" (https://hadoop.apache.org/docs/r1.0.4/hdfs_design.html) that HDFS provides interfaces for applications to move themselves closer to where the data is located. What are these interfaces and where they are in the source code? Is there any manual for the interfaces? Regards, Mahmood
