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https://issues.apache.org/jira/browse/HADOOP-1985?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12564165#action_12564165
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Devaraj Das commented on HADOOP-1985:
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bq. This implementation could introduce more delay in the Namenode startup
time. Maybe the ResolutionThread can invoke the resolution script with a bunch
of datanodes (rather than one datanode at a time).
I don't think this is going to be that big a deal if the implementation of the
resolution is efficient. At least for now, I'd like to keep the resolve method
take one datanode at a time.
bq. When the ResolutionThread operates on a datanode, it updates the
networkLocation field of the datanode. This means that if a datanode is used
before the resolution thread gets to it, it might return a default location,
but this is ok and acceptable. If we adopt this approach, then the Resolution
could be done much lazily, in fact, even towards the end of the SafeMode
period, thereby reducing namenode restart times.
This has a potential side effect .. for e.g., in the beginning of the namenode
run some dfs blocks will appear to be not present on all the racks & so will it
try to re-replicate, etc. ? But that can be special cased as we discussed
offline.
> Abstract node to switch mapping into a topology service class used by
> namenode and jobtracker
> ---------------------------------------------------------------------------------------------
>
> Key: HADOOP-1985
> URL: https://issues.apache.org/jira/browse/HADOOP-1985
> Project: Hadoop Core
> Issue Type: New Feature
> Components: dfs, mapred
> Reporter: eric baldeschwieler
> Assignee: Devaraj Das
> Fix For: 0.17.0
>
> Attachments: 1985.new.patch, 1985.v1.patch, 1985.v10.patch,
> 1985.v11.patch, 1985.v2.patch, 1985.v3.patch, 1985.v4.patch, 1985.v5.patch,
> 1985.v6.patch, 1985.v9.patch, jobinprogress.patch
>
>
> In order to implement switch locality in MapReduce, we need to have switch
> location in both the namenode and job tracker. Currently the namenode asks
> the data nodes for this info and they run a local script to answer this
> question. In our environment and others that I know of there is no reason to
> push this to each node. It is easier to maintain a centralized script that
> maps node DNS names to switch strings.
> I propose that we build a new class that caches known DNS name to switch
> mappings and invokes a loadable class or a configurable system call to
> resolve unknown DNS to switch mappings. We can then add this to the namenode
> to support the current block to switch mapping needs and simplify the data
> nodes. We can also add this same callout to the job tracker and then
> implement rack locality logic there without needing to chane the filesystem
> API or the split planning API.
> Not only is this the least intrusive path to building racklocal MR I can ID,
> it is also future compatible to future infrastructures that may derive
> topology on the fly, etc, etc...
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