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    E-MapReduce:FAQ

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    E-MapReduce:FAQ

    Last Updated:Mar 26, 2026

    This FAQ covers common issues and configuration tasks for Alluxio on E-MapReduce (EMR).

    Why does "No FileSystem for scheme: alluxio" appear?

    This error occurs when you add Alluxio to an existing EMR cluster but do not restart the service afterward. Alluxio must be restarted to load the Hadoop configurations.

    After adding Alluxio, restart the service. See Restart a service.

    This error does not occur if you select Alluxio from the optional services when creating the cluster.

    Why is Alluxio not working as expected?

    Start by identifying which node has the issue, then check the Alluxio service log on that node. In most cases, the log is at /mnt/disk1/log/alluxio/.

    How do I accelerate OSS data access with Alluxio caching?

    By default, HDFS is the Under File System (UFS) for Alluxio on EMR. To cache OSS data, mount an OSS directory as a mount point:

    alluxio fs mount --option fs.oss.accessKeyId=<OSS_ACCESS_KEY_ID> \
  --option fs.oss.accessKeySecret=<OSS_ACCESS_KEY_SECRET> \
  --option fs.oss.endpoint=<OSS_ENDPOINT> \
  /oss_dir <path>/
    Placeholder Description
    <OSS_ACCESS_KEY_ID> AccessKey ID for your Alibaba Cloud account. Get it from the AccessKey management console.
    <OSS_ACCESS_KEY_SECRET> AccessKey secret for your Alibaba Cloud account.
    <OSS_ENDPOINT> OSS endpoint in the format oss-<region>-internal.aliyuncs.com. Find it in the OSS console under bucket details. Use the internal endpoint (for example, oss-cn-shanghai-internal.aliyuncs.com). The EMR cluster must be in the same region as the OSS bucket.
    <path> OSS path in the format oss://<bucket-name>/<directory>.

    How do I modify Alluxio parameters?

    Two approaches are available: global configuration through the EMR console, or dynamic configuration at runtime.

    Global configuration

    Go to the Alluxio service page in the EMR console and modify the parameters there. Changes require a service restart to take effect. For details, see Manage parameters for services.

    Dynamic configuration

    Pass parameters at runtime using the -Dproperty=value format. The following examples all set the write type to CACHE_THROUGH.

    Alluxio shell

    Add -Dproperty=value flags directly to the command:

    alluxio fs copyFromLocal hello.txt /dir/tmp -Dalluxio.user.file.writetype.default=CACHE_THROUGH

    hello.txt is a local file and /dir/tmp is the destination directory in Alluxio. For other available commands, see Common commands.

    Spark jobs

    Add the -Dproperty=value flag to spark.driver.extraJavaOptions and spark.executor.extraJavaOptions:

    spark-submit \
  --conf 'spark.driver.extraJavaOptions=-Dalluxio.user.file.writetype.default=CACHE_THROUGH' \
  --conf 'spark.executor.extraJavaOptions=-Dalluxio.user.file.writetype.default=CACHE_THROUGH'

    MapReduce jobs

    Add the -Dproperty=value flag to hadoop jar or yarn jar:

    hadoop jar <HADOOP_HOME>/share/hadoop/mapreduce/hadoop-mapreduce-examples-x.x.x.jar wordcount \
  -Dalluxio.user.file.writetype.default=CACHE_THROUGH \
  -libjars /<PATH_TO_ALLUXIO>/client/alluxio-x.x.x.-client.jar \
  <path1> <path2>

    <path1> is the input path and <path2> is the output path. x.x.x is the JAR package version.