Mandatory: Short-Circuit Reads

Enabling short-circuit reads allows Impala to read local data directly from the file system. This removes the need to communicate through the DataNodes, improving performance. This setting also minimizes the number of additional copies of data. Short-circuit reads requires libhadoop.so (the Hadoop Native Library) to be accessible to both the server and the client. libhadoop.so is not available if you have installed from a tarball. You must install from an .rpm, .deb, or parcel to use short-circuit local reads.

  Note: If you use Cloudera Manager, you can enable short-circuit reads through a checkbox in the user interface and that setting takes effect for Impala as well.

Cloudera strongly recommends using Impala with CDH 4.2 or higher, ideally the latest 4.x release. Impala does support short-circuit reads with CDH 4.1, but for best performance, upgrade to CDH 4.3 or higher. The process of configuring short-circuit reads varies according to which version of CDH you are using. Choose the procedure that is appropriate for your environment.

To configure DataNodes for short-circuit reads with CDH 4.2 or later:

1. On all Impala nodes, configure the following properties in hdfs-site.xml as shown:

   <property>
       <name>dfs.client.read.shortcircuit</name>
       <value>true</value>
   </property>
   
   <property>
       <name>dfs.domain.socket.path</name>
       <value>/var/run/hadoop-hdfs/dn._PORT</value>
   </property>
   
   <property>
       <name>dfs.client.file-block-storage-locations.timeout.millis</name>
       <value>10000</value>
   </property>

     Note: The text _PORT appears just as shown; you do not need to substitute a number.

2. If /var/run/hadoop-hdfs/ is group-writable, make sure its group is root.

     Note: If you are also going to enable block location tracking, you can skip copying configuration files and restarting DataNodes and go straight to Optional: Block Location Tracking. Configuring short-circuit reads and block location tracking require the same process of copying files and restarting services, so you can complete that process once when you have completed all configuration changes. Whether you copy files and restart services now or during configuring block location tracking, short-circuit reads are not enabled until you complete those final steps.
3. Copy the client core-site.xml and hdfs-site.xml configuration files from the Hadoop configuration directory to the Impala configuration directory. The default Impala configuration location is /etc/impala/conf.
4. After applying these changes, restart all DataNodes.

To configure DataNodes for short-circuit reads with CDH 4.1:

  Note: Cloudera strongly recommends using Impala with CDH 4.2 or higher, ideally the latest 4.x release. Impala does support short-circuit reads with CDH 4.1, but for best performance, upgrade to CDH 4.3 or higher. The process of configuring short-circuit reads varies according to which version of CDH you are using. Choose the procedure that is appropriate for your environment.

1. Enable short-circuit reads by adding settings to the Impala core-site.xml file.
   • If you installed Impala using Cloudera Manager, short-circuit reads should be properly configured, but you can review the configuration by checking the contents of the core-site.xml file, which is installed at /etc/impala/conf by default.
   • If you installed using packages, instead of using Cloudera Manager, create the core-site.xml file. This can be easily done by copying the core-site.xml client configuration file from another machine that is running Hadoop services. This file must be copied to the Impala configuration directory. The Impala configuration directory is set by the IMPALA_CONF_DIR environment variable and is by default /etc/impala/conf. To confirm the Impala configuration directory, check the IMPALA_CONF_DIR environment variable value.
       Note: If the Impala configuration directory does not exist, create it and then add the core-site.xml file.
   Add the following to the core-site.xml file:

   <property>
      <name>dfs.client.read.shortcircuit</name>
      <value>true</value>
   </property>

     Note: For an installation managed by Cloudera Manager, specify these settings in the Impala dialogs, in the options field for HDFS. In Cloudera Manager 4, these fields are labelled Safety Valve; in Cloudera Manager 5, they are called Advanced Configuration Snippet
2. For each DataNode, enable access by adding the following to the hdfs-site.xml file:

   <property>
      <name>dfs.client.use.legacy.blockreader.local</name>
      <value>true</value>
   </property>
   
   <property>
      <name>dfs.datanode.data.dir.perm</name>
      <value>750</value>
   </property>
   
   <property>
      <name>dfs.block.local-path-access.user</name>
      <value>impala</value>
   </property>
   
   <property>
       <name>dfs.client.file-block-storage-locations.timeout.millis</name>
       <value>10000</value>
   </property>

     Note: In the preceding example, the dfs.block.local-path-access.user is the user running the impalad process. By default, that account is impala.
3. Use  usermod  to add users requiring local block access to the appropriate HDFS group. For example, if you assigned impala to the dfs.block.local-path-access.user  property, you would add impala  to the hadoop HDFS group:

   $ usermod -a -G hadoop impala

     Note: The default HDFS group is hadoop, but it is possible to have an environment configured to use an alternate group. To find the configured HDFS group name using the Cloudera Manager admin console, click Services and click HDFS. Click the  Configuration tab. Under Service-Wide, click Advanced  in the left column. The  Shared Hadoop Group Name  property contains the group name.
     Note: If you are going to enable block location tracking, you can skip copying configuration files and restarting DataNodes and go straight to Mandatory: Block Location Tracking. Configuring short-circuit reads and block location tracking require the same process of copying files and restarting services, so you can complete that process once when you have completed all configuration changes. Whether you copy files and restart services now or during configuring block location tracking, short-circuit reads are not enabled until you complete those final steps.
4. Copy the client core-site.xml and hdfs-site.xml configuration files from the Hadoop configuration directory to the Impala configuration directory. The default Impala configuration location is /etc/impala/conf.
5. After applying these changes, restart all DataNodes.

Mandatory: Block Location Tracking

Enabling block location metadata allows Impala to know which disk data blocks are located on, allowing better utilization of the underlying disks. Impala will not start unless this setting is enabled.

To enable block location tracking:

1. For each DataNode, adding the following to the hdfs-site.xml file:

   <property>
     <name>dfs.datanode.hdfs-blocks-metadata.enabled</name>
     <value>true</value>
   </property> 

2. Copy the client core-site.xml and hdfs-site.xml configuration files from the Hadoop configuration directory to the Impala configuration directory. The default Impala configuration location is /etc/impala/conf.
3. After applying these changes, restart all DataNodes.

Optional: Native Checksumming

Enabling native checksumming causes Impala to use an optimized native library for computing checksums, if that library is available.

To enable native checksumming:

If you installed CDH from packages, the native checksumming library is installed and setup correctly. In such a case, no additional steps are required. Conversely, if you installed by other means, such as with tarballs, native checksumming may not be available due to missing shared objects. Finding the message "Unable to load native-hadoop library for your platform... using builtin-java classes where applicable" in the Impala logs indicates native checksumming may be unavailable. To enable native checksumming, you must build and install libhadoop.so (the Hadoop Native Library).