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From "Devaraj Das (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 06:47:13 GMT

    [ https://issues.apache.org/jira/browse/HDFS-2576?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13589252#comment-13589252

Devaraj Das commented on HDFS-2576:

Good discussion points. However, do note that this mechanism is best effort and if possible
I'd like to avoid major surgeries in HDFS for this to work. What is currently there in the
patch seems to be a simple approach and not too intrusive in the HDFS layer (and should make
the average failure cases better, and the worst cases may be unaffected). The case in point
here is HBase and maybe the only major user of the API introduced in the patch (and maybe
we can mark the API private/evolving with HBase as the audience..)

Even after implementing an ideal solution, it'd be still best effort in terms of locality
(imagine cases where the hdfs balancer ends up doing a not so good job since it has to deal
with practical physical/disk constraints). The hdfs balancer would have to be significantly
changed if we want to do this in the HDFS (offline discussions with [~sureshms] indicated)
since the balancer works with blocks and doesn't know which directory owns which blocks. For
now if the balancer (which is run manually for HDFS) is too problematic, we can change the
balancer implementation so that it doesn't move blocks belonging to certain directories (the
information of blocks to paths seems to be there).

BTW another point is that applications like HBase may sometimes want to control the placement
of blocks for multi-tenancy reasons, load-balancing reasons, etc.

Maybe, at some point both mechanisms (client driven and namenode policy) of having favored
nodes in the HDFS might be required (policy, if configured, always wins in case of conflicts).
> 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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