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https://issues.apache.org/jira/browse/MAPREDUCE-5278?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13686060#comment-13686060
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Xi Fang commented on MAPREDUCE-5278:
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Thanks Bikas for your comments. For your question : "Is the following code
(marked below) continuing to copy stuff to the default fs (fs) when the newPath
points to a different filesystem?:
No. Basically, the original code does this: If JT staging dir is not on the
default FS (for example, in our context it is ASV), copyRemoteFiles() will copy
files in ASV to JT. Note that these files are specified using generic options.
After our change, when ASV is marked as "accessible" by specifying
"mapreduce.client.accessible.remote.schemes", copyRemoteFiles() won't copy the
files in ASV to the jobtracker. It just directly returns the path of that
file, denoted by "newPath". In addition, no copy operation would happen in
addArchiveToClassPath().
> Distributed cache is broken when JT staging dir is not on the default FS
> ------------------------------------------------------------------------
>
> Key: MAPREDUCE-5278
> URL: https://issues.apache.org/jira/browse/MAPREDUCE-5278
> Project: Hadoop Map/Reduce
> Issue Type: Bug
> Components: distributed-cache
> Affects Versions: 1-win
> Environment: Windows
> Reporter: Xi Fang
> Assignee: Xi Fang
> Fix For: 1-win
>
> Attachments: MAPREDUCE-5278.2.patch, MAPREDUCE-5278.patch
>
>
> Today, the JobTracker staging dir ("mapreduce.jobtracker.staging.root.dir) is
> set to point to HDFS, even though other file systems (e.g. Amazon S3 file
> system and Windows ASV file system) are the default file systems.
> For ASV, this config was chosen and there are a few reasons why:
> 1. To prevent leak of the storage account credentials to the user's storage
> account;
> 2. It uses HDFS for the transient job files what is good for two reasons – a)
> it does not flood the user's storage account with irrelevant data/files b) it
> leverages HDFS locality for small files
> However, this approach conflicts with how distributed cache caching works,
> completely negating the feature's functionality.
> When files are added to the distributed cache (thru files/achieves/libjars
> hadoop generic options), they are copied to the job tracker staging dir only
> if they reside on a file system different that the jobtracker's. Later on,
> this path is used as a "key" to cache the files locally on the tasktracker's
> machine, and avoid localization (download/unzip) of the distributed cache
> files if they are already localized.
> In this configuration the caching is completely disabled and we always end up
> copying dist cache files to the job tracker's staging dir first and
> localizing them on the task tracker machine second.
> This is especially not good for Oozie scenarios as Oozie uses dist cache to
> populate Hive/Pig jars throughout the cluster.
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