Xi Fang created MAPREDUCE-5278:
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Summary: Perf: 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
Today, we set the JobTracker staging dir
("mapreduce.jobtracker.staging.root.dir) to point to HDFS even though ASV is
the default file system. There are a few reason why this config was chosen:
To prevent leak of the storage account creds to the user's storage account
(IOW, keep job.xml in the cluster). This is needed until HADOOP-444 is fixed.
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 our configuration the caching is completely disabled and we always end up
copying dist cache files to the JT staging dir first and localizing them on the
tasktracker machine second.
This is especially not good for Oozie scenarios as Oozie uses dist cache to
populate Hive/Pig jars throughout the cluster.
Easy workaround is to config mapreduce.jobtracker.staging.root.dir in
mapred-site.xml to be on the default FS.
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