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ba3e587036
Added detail usage notes for REPLICA_PREFERENCE. Change-Id: If38f9c881f553568c2516ecc23ec501f23ee1f28 Reviewed-on: http://gerrit.cloudera.org:8080/9877 Reviewed-by: John Russell <jrussell@cloudera.com> Tested-by: Impala Public Jenkins
94 lines
3.1 KiB
XML
94 lines
3.1 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!--
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Licensed to the Apache Software Foundation (ASF) under one
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or more contributor license agreements. See the NOTICE file
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distributed with this work for additional information
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regarding copyright ownership. The ASF licenses this file
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to you under the Apache License, Version 2.0 (the
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"License"); you may not use this file except in compliance
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with the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing,
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software distributed under the License is distributed on an
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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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KIND, either express or implied. See the License for the
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specific language governing permissions and limitations
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under the License.
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-->
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<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
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<concept id="replica_preference" rev="2.7.0">
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<title>REPLICA_PREFERENCE Query Option (<keyword keyref="impala27"/> or higher only)</title>
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<titlealts audience="PDF">
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<navtitle>REPLICA_PREFERENCE</navtitle>
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</titlealts>
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<prolog>
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<metadata>
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<data name="Category" value="Impala"/>
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<data name="Category" value="Impala Query Options"/>
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<data name="Category" value="Developers"/>
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<data name="Category" value="Data Analysts"/>
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</metadata>
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</prolog>
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<conbody>
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<p rev="2.7.0">
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<indexterm audience="hidden">REPLICA_PREFERENCE query option</indexterm>
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</p>
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<p>
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The <codeph>REPLICA_PREFERENCE</codeph> query option lets you distribute the work more
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evenly if hotspots and bottlenecks persist. It causes the access cost of all replicas of a
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data block to be considered equal to or worse than the configured value. This allows
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Impala to schedule reads to suboptimal replicas (e.g. local in the presence of cached
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ones) in order to distribute the work across more executor nodes.
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</p>
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<p>
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Allowed values are: <codeph>CACHE_LOCAL</codeph> (<codeph>0</codeph>),
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<codeph>DISK_LOCAL</codeph> (<codeph>2</codeph>), <codeph>REMOTE</codeph>
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(<codeph>4</codeph>)
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</p>
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<p>
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<b>Type:</b> Enum
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</p>
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<p>
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<b>Default:</b> <codeph>CACHE_LOCAL (0)</codeph>
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</p>
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<p conref="../shared/impala_common.xml#common/added_in_270"/>
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<p>
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<b>Usage Notes:</b>
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</p>
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<p>
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By default Impala selects the best replica it can find in terms of access cost. The
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preferred order is cached, local, and remote. With <codeph>REPLICA_PREFERENCE</codeph>,
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the preference of all replicas are capped at the selected value. For example, when
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<codeph>REPLICA_PREFERENCE</codeph> is set to <codeph>DISK_LOCAL</codeph>, cached and
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local replicas are treated with the equal preference. When set to
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<codeph>REMOTE</codeph>, all three types of replicas, cached, local, remote, are treated
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with equal preference.
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</p>
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<p conref="../shared/impala_common.xml#common/related_info"/>
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<p>
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<xref href="impala_perf_hdfs_caching.xml#hdfs_caching"/>,
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<xref href="impala_schedule_random_replica.xml#schedule_random_replica"/>
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</p>
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</conbody>
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</concept>
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