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From "Jonathan Hsieh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-2576) Namenode should have a favored nodes hint to enable clients to have control over block placement.
Date Thu, 28 Feb 2013 13:57:16 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13589542#comment-13589542

Jonathan Hsieh commented on HDFS-2576:

[~devaraj], [~saint.ack@gmail.com] I basically agree with short-term assessment of incremental
work for incremental benefit, I just didn't see any discussion about alternatives and what
potential ideals are. 

If we eventually went with the "ideal" approach, HDFS would still need this hint API internally
to HDFS so it is likely an incremental step.  I don't think that hbase would be the only major
user of this feature -- [~atm] mentions "file formats which will be used in joins" -- and
other applications would want low-latency access in the face of failures will want this kind
of control as well.  

I don't know all the internals of the hfds balancer, but it would seem that if it a block
contained the dirent, the affinity group info should be available.  We'd probably need to
define an interface and semantics where files created under a subdir inherit the affinity
of its parent and/or a mechanism to modify affinity groups.

> Namenode should have a favored nodes hint to enable clients to have control over block
> -------------------------------------------------------------------------------------------------
>                 Key: HDFS-2576
>                 URL: https://issues.apache.org/jira/browse/HDFS-2576
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>            Reporter: Pritam Damania
>         Attachments: hdfs-2576-1.txt
> Sometimes Clients like HBase are required to dynamically compute the datanodes it wishes
to place the blocks for a file for higher level of locality. For this purpose there is a need
of a way to give the Namenode a hint in terms of a favoredNodes parameter about the locations
where the client wants to put each block. The proposed solution is a favored nodes parameter
in the addBlock() method and in the create() file method to enable the clients to give the
hints to the NameNode about the locations of each replica of the block. Note that this would
be just a hint and finally the NameNode would look at disk usage, datanode load etc. and decide
whether it can respect the hints or not.

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