HDFS-3926. QJM: Add user documentation for QJM. Contributed by Aaron T. Myers.
git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-3077@1384595 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Aaron Myers
parent
40d56f1741
commit
853db9ec24
@ -72,3 +72,5 @@ HDFS-3906. QJM: quorum timeout on failover with large log segment (todd)
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HDFS-3840. JournalNodes log JournalNotFormattedException backtrace error before being formatted (todd)
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HDFS-3894. QJM: testRecoverAfterDoubleFailures can be flaky due to IPC client caching (todd)
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HDFS-3926. QJM: Add user documentation for QJM. (atm)
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@ -54,7 +54,8 @@
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</menu>
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<menu name="HDFS" inherit="top">
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<item name="High Availability" href="hadoop-yarn/hadoop-yarn-site/HDFSHighAvailability.html"/>
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<item name="High Availability With QJM" href="hadoop-yarn/hadoop-yarn-site/HDFSHighAvailabilityWithQJM.html"/>
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<item name="High Availability With NFS" href="hadoop-yarn/hadoop-yarn-site/HDFSHighAvailabilityWithNFS.html"/>
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<item name="Federation" href="hadoop-yarn/hadoop-yarn-site/Federation.html"/>
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<item name="WebHDFS REST API" href="hadoop-yarn/hadoop-yarn-site/WebHDFS.html"/>
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<item name="HttpFS Gateway" href="hadoop-hdfs-httpfs/index.html"/>
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@ -25,12 +25,21 @@ HDFS High Availability
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* {Purpose}
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This guide provides an overview of the HDFS High Availability (HA) feature and
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how to configure and manage an HA HDFS cluster.
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how to configure and manage an HA HDFS cluster, using NFS for the shared
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storage required by the NameNodes.
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This document assumes that the reader has a general understanding of
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general components and node types in an HDFS cluster. Please refer to the
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HDFS Architecture guide for details.
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* {Note: Using the Quorum Journal Manager or Conventional Shared Storage}
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This guide discusses how to configure and use HDFS HA using a shared NFS
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directory to share edit logs between the Active and Standby NameNodes. For
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information on how to configure HDFS HA using the Quorum Journal Manager
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instead of NFS, please see {{{./HDFSHighAvailabilityWithQJM.html}this
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alternative guide.}}
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* {Background}
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Prior to Hadoop 2.0.0, the NameNode was a single point of failure (SPOF) in
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@ -297,7 +306,7 @@ HDFS High Availability
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</property>
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<property>
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<name>dfs.ha.fencing.ssh.connect-timeout</name>
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<value>
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<value>30000</value>
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</property>
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---
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@ -375,17 +384,22 @@ HDFS High Availability
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** Deployment details
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After all of the necessary configuration options have been set, one must
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initially synchronize the two HA NameNodes' on-disk metadata. If you are
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setting up a fresh HDFS cluster, you should first run the format command (<hdfs
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namenode -format>) on one of NameNodes. If you have already formatted the
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NameNode, or are converting a non-HA-enabled cluster to be HA-enabled, you
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should now copy over the contents of your NameNode metadata directories to
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the other, unformatted NameNode using <scp> or a similar utility. The location
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of the directories containing the NameNode metadata are configured via the
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configuration options <<dfs.namenode.name.dir>> and/or
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<<dfs.namenode.edits.dir>>. At this time, you should also ensure that the
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shared edits dir (as configured by <<dfs.namenode.shared.edits.dir>>) includes
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all recent edits files which are in your NameNode metadata directories.
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initially synchronize the two HA NameNodes' on-disk metadata.
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* If you are setting up a fresh HDFS cluster, you should first run the format
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command (<hdfs namenode -format>) on one of NameNodes.
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* If you have already formatted the NameNode, or are converting a
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non-HA-enabled cluster to be HA-enabled, you should now copy over the
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contents of your NameNode metadata directories to the other, unformatted
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NameNode by running the command "<hdfs namenode -bootstrapStandby>" on the
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unformatted NameNode. Running this command will also ensure that the shared
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edits directory (as configured by <<dfs.namenode.shared.edits.dir>>) contains
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sufficient edits transactions to be able to start both NameNodes.
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* If you are converting a non-HA NameNode to be HA, you should run the
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command "<hdfs -initializeSharedEdits>", which will initialize the shared
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edits directory with the edits data from the local NameNode edits directories.
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At this point you may start both of your HA NameNodes as you normally would
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start a NameNode.
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@ -863,4 +877,4 @@ $ zkCli.sh create /ledgers/available 0
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3) Auto-Recovery of storage node failures. Work inprogress
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{{{https://issues.apache.org/jira/browse/BOOKKEEPER-237 }BOOKKEEPER-237}}.
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Currently we have the tools to manually recover the data from failed storage nodes.
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Currently we have the tools to manually recover the data from failed storage nodes.
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@ -0,0 +1,767 @@
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~~ Licensed under the Apache License, Version 2.0 (the "License");
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~~ you may not use this file except in compliance with the License.
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~~ You may obtain a copy of the License at
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~~
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~~ http://www.apache.org/licenses/LICENSE-2.0
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~~
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~~ Unless required by applicable law or agreed to in writing, software
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~~ distributed under the License is distributed on an "AS IS" BASIS,
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~~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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~~ See the License for the specific language governing permissions and
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~~ limitations under the License. See accompanying LICENSE file.
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---
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Hadoop Distributed File System-${project.version} - High Availability
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---
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---
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${maven.build.timestamp}
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HDFS High Availability Using the Quorum Journal Manager
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\[ {{{./index.html}Go Back}} \]
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%{toc|section=1|fromDepth=0}
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* {Purpose}
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This guide provides an overview of the HDFS High Availability (HA) feature
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and how to configure and manage an HA HDFS cluster, using the Quorum Journal
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Manager (QJM) feature.
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This document assumes that the reader has a general understanding of
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general components and node types in an HDFS cluster. Please refer to the
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HDFS Architecture guide for details.
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* {Note: Using the Quorum Journal Manager or Conventional Shared Storage}
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This guide discusses how to configure and use HDFS HA using the Quorum
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Journal Manager (QJM) to share edit logs between the Active and Standby
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NameNodes. For information on how to configure HDFS HA using NFS for shared
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storage instead of the QJM, please see
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{{{./HDFSHighAvailabilityWithNFS.html}this alternative guide.}}
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* {Background}
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Prior to Hadoop 2.0.0, the NameNode was a single point of failure (SPOF) in
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an HDFS cluster. Each cluster had a single NameNode, and if that machine or
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process became unavailable, the cluster as a whole would be unavailable
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until the NameNode was either restarted or brought up on a separate machine.
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This impacted the total availability of the HDFS cluster in two major ways:
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* In the case of an unplanned event such as a machine crash, the cluster would
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be unavailable until an operator restarted the NameNode.
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* Planned maintenance events such as software or hardware upgrades on the
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NameNode machine would result in windows of cluster downtime.
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The HDFS High Availability feature addresses the above problems by providing
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the option of running two redundant NameNodes in the same cluster in an
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Active/Passive configuration with a hot standby. This allows a fast failover to
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a new NameNode in the case that a machine crashes, or a graceful
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administrator-initiated failover for the purpose of planned maintenance.
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* {Architecture}
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In a typical HA cluster, two separate machines are configured as NameNodes.
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At any point in time, exactly one of the NameNodes is in an <Active> state,
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and the other is in a <Standby> state. The Active NameNode is responsible
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for all client operations in the cluster, while the Standby is simply acting
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as a slave, maintaining enough state to provide a fast failover if
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necessary.
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In order for the Standby node to keep its state synchronized with the Active
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node, both nodes communicate with a group of separate daemons called
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"JournalNodes" (JNs). When any namespace modification is performed by the
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Active node, it durably logs a record of the modification to a majority of
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these JNs. The Standby node is capable of reading the edits from the JNs, and
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is constantly watching them for changes to the edit log. As the Standby Node
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sees the edits, it applies them to its own namespace. In the event of a
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failover, the Standby will ensure that it has read all of the edits from the
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JounalNodes before promoting itself to the Active state. This ensures that the
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namespace state is fully synchronized before a failover occurs.
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In order to provide a fast failover, it is also necessary that the Standby node
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have up-to-date information regarding the location of blocks in the cluster.
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In order to achieve this, the DataNodes are configured with the location of
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both NameNodes, and send block location information and heartbeats to both.
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It is vital for the correct operation of an HA cluster that only one of the
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NameNodes be Active at a time. Otherwise, the namespace state would quickly
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diverge between the two, risking data loss or other incorrect results. In
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order to ensure this property and prevent the so-called "split-brain scenario,"
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the JournalNodes will only ever allow a single NameNode to be a writer at a
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time. During a failover, the NameNode which is to become active will simply
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take over the role of writing to the JournalNodes, which will effectively
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prevent the other NameNode from continuing in the Active state, allowing the
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new Active to safely proceed with failover.
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* {Hardware resources}
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In order to deploy an HA cluster, you should prepare the following:
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* <<NameNode machines>> - the machines on which you run the Active and
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Standby NameNodes should have equivalent hardware to each other, and
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equivalent hardware to what would be used in a non-HA cluster.
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* <<JournalNode machines>> - the machines on which you run the JournalNodes.
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The JournalNode daemon is relatively lightweight, so these daemons may
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reasonably be collocated on machines with other Hadoop daemons, for example
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NameNodes, the JobTracker, or the YARN ResourceManager. <<Note:>> There
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must be at least 3 JournalNode daemons, since edit log modifications must be
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written to a majority of JNs. This will allow the system to tolerate the
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failure of a single machine. You may also run more than 3 JournalNodes, but
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in order to actually increase the number of failures the system can tolerate,
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you should run an odd number of JNs, (i.e. 3, 5, 7, etc.). Note that when
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running with N JournalNodes, the system can tolerate at most (N - 1) / 2
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failures and continue to function normally.
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Note that, in an HA cluster, the Standby NameNode also performs checkpoints of
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the namespace state, and thus it is not necessary to run a Secondary NameNode,
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CheckpointNode, or BackupNode in an HA cluster. In fact, to do so would be an
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error. This also allows one who is reconfiguring a non-HA-enabled HDFS cluster
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to be HA-enabled to reuse the hardware which they had previously dedicated to
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the Secondary NameNode.
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* {Deployment}
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** Configuration overview
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Similar to Federation configuration, HA configuration is backward compatible
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and allows existing single NameNode configurations to work without change.
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The new configuration is designed such that all the nodes in the cluster may
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have the same configuration without the need for deploying different
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configuration files to different machines based on the type of the node.
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Like HDFS Federation, HA clusters reuse the <<<nameservice ID>>> to identify a
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single HDFS instance that may in fact consist of multiple HA NameNodes. In
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addition, a new abstraction called <<<NameNode ID>>> is added with HA. Each
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distinct NameNode in the cluster has a different NameNode ID to distinguish it.
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To support a single configuration file for all of the NameNodes, the relevant
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configuration parameters are suffixed with the <<nameservice ID>> as well as
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the <<NameNode ID>>.
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** Configuration details
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To configure HA NameNodes, you must add several configuration options to your
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<<hdfs-site.xml>> configuration file.
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The order in which you set these configurations is unimportant, but the values
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you choose for <<dfs.nameservices>> and
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<<dfs.ha.namenodes.[nameservice ID]>> will determine the keys of those that
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follow. Thus, you should decide on these values before setting the rest of the
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configuration options.
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* <<dfs.nameservices>> - the logical name for this new nameservice
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Choose a logical name for this nameservice, for example "mycluster", and use
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this logical name for the value of this config option. The name you choose is
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arbitrary. It will be used both for configuration and as the authority
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component of absolute HDFS paths in the cluster.
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<<Note:>> If you are also using HDFS Federation, this configuration setting
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should also include the list of other nameservices, HA or otherwise, as a
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comma-separated list.
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----
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<property>
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<name>dfs.nameservices</name>
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<value>mycluster</value>
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</property>
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----
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* <<dfs.ha.namenodes.[nameservice ID]>> - unique identifiers for each NameNode in the nameservice
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Configure with a list of comma-separated NameNode IDs. This will be used by
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DataNodes to determine all the NameNodes in the cluster. For example, if you
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used "mycluster" as the nameservice ID previously, and you wanted to use "nn1"
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and "nn2" as the individual IDs of the NameNodes, you would configure this as
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such:
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----
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<property>
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<name>dfs.ha.namenodes.mycluster</name>
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<value>nn1,nn2</value>
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</property>
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----
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<<Note:>> Currently, only a maximum of two NameNodes may be configured per
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nameservice.
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* <<dfs.namenode.rpc-address.[nameservice ID].[name node ID]>> - the fully-qualified RPC address for each NameNode to listen on
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For both of the previously-configured NameNode IDs, set the full address and
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IPC port of the NameNode processs. Note that this results in two separate
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configuration options. For example:
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----
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<property>
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<name>dfs.namenode.rpc-address.mycluster.nn1</name>
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<value>machine1.example.com:8020</value>
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</property>
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<property>
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<name>dfs.namenode.rpc-address.mycluster.nn2</name>
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<value>machine2.example.com:8020</value>
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</property>
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----
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<<Note:>> You may similarly configure the "<<servicerpc-address>>" setting if
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you so desire.
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* <<dfs.namenode.http-address.[nameservice ID].[name node ID]>> - the fully-qualified HTTP address for each NameNode to listen on
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Similarly to <rpc-address> above, set the addresses for both NameNodes' HTTP
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servers to listen on. For example:
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----
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<property>
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<name>dfs.namenode.http-address.mycluster.nn1</name>
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<value>machine1.example.com:50070</value>
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</property>
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<property>
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<name>dfs.namenode.http-address.mycluster.nn2</name>
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<value>machine2.example.com:50070</value>
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</property>
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----
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<<Note:>> If you have Hadoop's security features enabled, you should also set
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the <https-address> similarly for each NameNode.
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* <<dfs.namenode.shared.edits.dir>> - the URI which identifies the group of JNs where the NameNodes will write/read edits
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This is where one configures the addresses of the JournalNodes which provide
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the shared edits storage, written to by the Active nameNode and read by the
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Standby NameNode to stay up-to-date with all the file system changes the Active
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NameNode makes. Though you must specify several JournalNode addresses,
|
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<<you should only configure one of these URIs.>> The URI should be of the form:
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"qjournal://<host1:port1>;<host2:port2>;<host3:port3>/<journalId>". The Journal
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ID is a unique identifier for this nameservice, which allows a single set of
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JournalNodes to provide storage for multiple federated namesystems. Though not
|
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a requirement, it's a good idea to reuse the nameservice ID for the journal
|
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identifier.
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For example, if the JournalNodes for this cluster were running on the
|
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machines "node1.example.com", "node2.example.com", and "node3.example.com" and
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the nameservice ID were "mycluster", you would use the following as the value
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for this setting (the default port for the JournalNode is 8485):
|
||||
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||||
----
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<property>
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<name>dfs.namenode.shared.edits.dir</name>
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<value>qjournal://node1.example.com:8485;node2.example.com:8485;node3.example.com:8485/mycluster</value>
|
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</property>
|
||||
----
|
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* <<dfs.client.failover.proxy.provider.[nameservice ID]>> - the Java class that HDFS clients use to contact the Active NameNode
|
||||
|
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Configure the name of the Java class which will be used by the DFS Client to
|
||||
determine which NameNode is the current Active, and therefore which NameNode is
|
||||
currently serving client requests. The only implementation which currently
|
||||
ships with Hadoop is the <<ConfiguredFailoverProxyProvider>>, so use this
|
||||
unless you are using a custom one. For example:
|
||||
|
||||
----
|
||||
<property>
|
||||
<name>dfs.client.failover.proxy.provider.mycluster</name>
|
||||
<value>org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider</value>
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||||
</property>
|
||||
----
|
||||
|
||||
* <<dfs.ha.fencing.methods>> - a list of scripts or Java classes which will be used to fence the Active NameNode during a failover
|
||||
|
||||
It is desirable for correctness of the system that only one NameNode be in
|
||||
the Active state at any given time. <<Importantly, when using the Quorum
|
||||
Journal Manager, only one NameNode will ever be allowed to write to the
|
||||
JournalNodes, so there is no potential for corrupting the file system metadata
|
||||
from a split-brain scenario.>> However, when a failover occurs, it is still
|
||||
possible that the previous Active NameNode could serve read requests to
|
||||
clients, which may be out of date until that NameNode shuts down when trying to
|
||||
write to the JournalNodes. For this reason, it is still desirable to configure
|
||||
some fencing methods even when using the Quorum Journal Manager. However, to
|
||||
improve the availability of the system in the event the fencing mechanisms
|
||||
fail, it is advisable to configure a fencing method which is guaranteed to
|
||||
return success as the last fencing method in the list. Note that if you choose
|
||||
to use no actual fencing methods, you still must configure something for this
|
||||
setting, for example "<<<shell(/bin/true)>>>".
|
||||
|
||||
The fencing methods used during a failover are configured as a
|
||||
carriage-return-separated list, which will be attempted in order until one
|
||||
indicates that fencing has succeeded. There are two methods which ship with
|
||||
Hadoop: <shell> and <sshfence>. For information on implementing your own custom
|
||||
fencing method, see the <org.apache.hadoop.ha.NodeFencer> class.
|
||||
|
||||
* <<sshfence>> - SSH to the Active NameNode and kill the process
|
||||
|
||||
The <sshfence> option SSHes to the target node and uses <fuser> to kill the
|
||||
process listening on the service's TCP port. In order for this fencing option
|
||||
to work, it must be able to SSH to the target node without providing a
|
||||
passphrase. Thus, one must also configure the
|
||||
<<dfs.ha.fencing.ssh.private-key-files>> option, which is a
|
||||
comma-separated list of SSH private key files. For example:
|
||||
|
||||
---
|
||||
<property>
|
||||
<name>dfs.ha.fencing.methods</name>
|
||||
<value>sshfence</value>
|
||||
</property>
|
||||
|
||||
<property>
|
||||
<name>dfs.ha.fencing.ssh.private-key-files</name>
|
||||
<value>/home/exampleuser/.ssh/id_rsa</value>
|
||||
</property>
|
||||
---
|
||||
|
||||
Optionally, one may configure a non-standard username or port to perform the
|
||||
SSH. One may also configure a timeout, in milliseconds, for the SSH, after
|
||||
which this fencing method will be considered to have failed. It may be
|
||||
configured like so:
|
||||
|
||||
---
|
||||
<property>
|
||||
<name>dfs.ha.fencing.methods</name>
|
||||
<value>sshfence([[username][:port]])</value>
|
||||
</property>
|
||||
<property>
|
||||
<name>dfs.ha.fencing.ssh.connect-timeout</name>
|
||||
<value>30000</value>
|
||||
</property>
|
||||
---
|
||||
|
||||
* <<shell>> - run an arbitrary shell command to fence the Active NameNode
|
||||
|
||||
The <shell> fencing method runs an arbitrary shell command. It may be
|
||||
configured like so:
|
||||
|
||||
---
|
||||
<property>
|
||||
<name>dfs.ha.fencing.methods</name>
|
||||
<value>shell(/path/to/my/script.sh arg1 arg2 ...)</value>
|
||||
</property>
|
||||
---
|
||||
|
||||
The string between '(' and ')' is passed directly to a bash shell and may not
|
||||
include any closing parentheses.
|
||||
|
||||
The shell command will be run with an environment set up to contain all of the
|
||||
current Hadoop configuration variables, with the '_' character replacing any
|
||||
'.' characters in the configuration keys. The configuration used has already had
|
||||
any namenode-specific configurations promoted to their generic forms -- for example
|
||||
<<dfs_namenode_rpc-address>> will contain the RPC address of the target node, even
|
||||
though the configuration may specify that variable as
|
||||
<<dfs.namenode.rpc-address.ns1.nn1>>.
|
||||
|
||||
Additionally, the following variables referring to the target node to be fenced
|
||||
are also available:
|
||||
|
||||
*-----------------------:-----------------------------------+
|
||||
| $target_host | hostname of the node to be fenced |
|
||||
*-----------------------:-----------------------------------+
|
||||
| $target_port | IPC port of the node to be fenced |
|
||||
*-----------------------:-----------------------------------+
|
||||
| $target_address | the above two, combined as host:port |
|
||||
*-----------------------:-----------------------------------+
|
||||
| $target_nameserviceid | the nameservice ID of the NN to be fenced |
|
||||
*-----------------------:-----------------------------------+
|
||||
| $target_namenodeid | the namenode ID of the NN to be fenced |
|
||||
*-----------------------:-----------------------------------+
|
||||
|
||||
These environment variables may also be used as substitutions in the shell
|
||||
command itself. For example:
|
||||
|
||||
---
|
||||
<property>
|
||||
<name>dfs.ha.fencing.methods</name>
|
||||
<value>shell(/path/to/my/script.sh --nameservice=$target_nameserviceid $target_host:$target_port)</value>
|
||||
</property>
|
||||
---
|
||||
|
||||
If the shell command returns an exit
|
||||
code of 0, the fencing is determined to be successful. If it returns any other
|
||||
exit code, the fencing was not successful and the next fencing method in the
|
||||
list will be attempted.
|
||||
|
||||
<<Note:>> This fencing method does not implement any timeout. If timeouts are
|
||||
necessary, they should be implemented in the shell script itself (eg by forking
|
||||
a subshell to kill its parent in some number of seconds).
|
||||
|
||||
* <<fs.defaultFS>> - the default path prefix used by the Hadoop FS client when none is given
|
||||
|
||||
Optionally, you may now configure the default path for Hadoop clients to use
|
||||
the new HA-enabled logical URI. If you used "mycluster" as the nameservice ID
|
||||
earlier, this will be the value of the authority portion of all of your HDFS
|
||||
paths. This may be configured like so, in your <<core-site.xml>> file:
|
||||
|
||||
---
|
||||
<property>
|
||||
<name>fs.defaultFS</name>
|
||||
<value>hdfs://mycluster</value>
|
||||
</property>
|
||||
---
|
||||
|
||||
|
||||
* <<dfs.journalnode.edits.dir>> - the path where the JournalNode daemon will store its local state
|
||||
|
||||
This is the absolute path on the JournalNode machines where the edits and
|
||||
other local state used by the JNs will be stored. You may only use a single
|
||||
path for this configuration. Redundancy for this data is provided by running
|
||||
multiple separate JournalNodes, or by configuring this directory on a
|
||||
locally-attached RAID array. For example:
|
||||
|
||||
---
|
||||
<property>
|
||||
<name>dfs.journalnode.edits.dir</name>
|
||||
<value>/path/to/journal/node/local/data</value>
|
||||
</property>
|
||||
---
|
||||
|
||||
** Deployment details
|
||||
|
||||
After all of the necessary configuration options have been set, you must
|
||||
start the JournalNode daemons on the set of machines where they will run. This
|
||||
can be done by running the command "<hdfs-daemon.sh journalnode>" and waiting
|
||||
for the daemon to start on each of the relevant machines.
|
||||
|
||||
Once the JournalNodes have been started, one must initially synchronize the
|
||||
two HA NameNodes' on-disk metadata.
|
||||
|
||||
* If you are setting up a fresh HDFS cluster, you should first run the format
|
||||
command (<hdfs namenode -format>) on one of NameNodes.
|
||||
|
||||
* If you have already formatted the NameNode, or are converting a
|
||||
non-HA-enabled cluster to be HA-enabled, you should now copy over the
|
||||
contents of your NameNode metadata directories to the other, unformatted
|
||||
NameNode by running the command "<hdfs namenode -bootstrapStandby>" on the
|
||||
unformatted NameNode. Running this command will also ensure that the
|
||||
JournalNodes (as configured by <<dfs.namenode.shared.edits.dir>>) contain
|
||||
sufficient edits transactions to be able to start both NameNodes.
|
||||
|
||||
* If you are converting a non-HA NameNode to be HA, you should run the
|
||||
command "<hdfs -initializeSharedEdits>", which will initialize the
|
||||
JournalNodes with the edits data from the local NameNode edits directories.
|
||||
|
||||
At this point you may start both of your HA NameNodes as you normally would
|
||||
start a NameNode.
|
||||
|
||||
You can visit each of the NameNodes' web pages separately by browsing to their
|
||||
configured HTTP addresses. You should notice that next to the configured
|
||||
address will be the HA state of the NameNode (either "standby" or "active".)
|
||||
Whenever an HA NameNode starts, it is initially in the Standby state.
|
||||
|
||||
** Administrative commands
|
||||
|
||||
Now that your HA NameNodes are configured and started, you will have access
|
||||
to some additional commands to administer your HA HDFS cluster. Specifically,
|
||||
you should familiarize yourself with all of the subcommands of the "<hdfs
|
||||
haadmin>" command. Running this command without any additional arguments will
|
||||
display the following usage information:
|
||||
|
||||
---
|
||||
Usage: DFSHAAdmin [-ns <nameserviceId>]
|
||||
[-transitionToActive <serviceId>]
|
||||
[-transitionToStandby <serviceId>]
|
||||
[-failover [--forcefence] [--forceactive] <serviceId> <serviceId>]
|
||||
[-getServiceState <serviceId>]
|
||||
[-checkHealth <serviceId>]
|
||||
[-help <command>]
|
||||
---
|
||||
|
||||
This guide describes high-level uses of each of these subcommands. For
|
||||
specific usage information of each subcommand, you should run "<hdfs haadmin
|
||||
-help <command>>".
|
||||
|
||||
* <<transitionToActive>> and <<transitionToStandby>> - transition the state of the given NameNode to Active or Standby
|
||||
|
||||
These subcommands cause a given NameNode to transition to the Active or Standby
|
||||
state, respectively. <<These commands do not attempt to perform any fencing,
|
||||
and thus should rarely be used.>> Instead, one should almost always prefer to
|
||||
use the "<hdfs haadmin -failover>" subcommand.
|
||||
|
||||
* <<failover>> - initiate a failover between two NameNodes
|
||||
|
||||
This subcommand causes a failover from the first provided NameNode to the
|
||||
second. If the first NameNode is in the Standby state, this command simply
|
||||
transitions the second to the Active state without error. If the first NameNode
|
||||
is in the Active state, an attempt will be made to gracefully transition it to
|
||||
the Standby state. If this fails, the fencing methods (as configured by
|
||||
<<dfs.ha.fencing.methods>>) will be attempted in order until one
|
||||
succeeds. Only after this process will the second NameNode be transitioned to
|
||||
the Active state. If no fencing method succeeds, the second NameNode will not
|
||||
be transitioned to the Active state, and an error will be returned.
|
||||
|
||||
* <<getServiceState>> - determine whether the given NameNode is Active or Standby
|
||||
|
||||
Connect to the provided NameNode to determine its current state, printing
|
||||
either "standby" or "active" to STDOUT appropriately. This subcommand might be
|
||||
used by cron jobs or monitoring scripts which need to behave differently based
|
||||
on whether the NameNode is currently Active or Standby.
|
||||
|
||||
* <<checkHealth>> - check the health of the given NameNode
|
||||
|
||||
Connect to the provided NameNode to check its health. The NameNode is capable
|
||||
of performing some diagnostics on itself, including checking if internal
|
||||
services are running as expected. This command will return 0 if the NameNode is
|
||||
healthy, non-zero otherwise. One might use this command for monitoring
|
||||
purposes.
|
||||
|
||||
<<Note:>> This is not yet implemented, and at present will always return
|
||||
success, unless the given NameNode is completely down.
|
||||
|
||||
* {Automatic Failover}
|
||||
|
||||
** Introduction
|
||||
|
||||
The above sections describe how to configure manual failover. In that mode,
|
||||
the system will not automatically trigger a failover from the active to the
|
||||
standby NameNode, even if the active node has failed. This section describes
|
||||
how to configure and deploy automatic failover.
|
||||
|
||||
** Components
|
||||
|
||||
Automatic failover adds two new components to an HDFS deployment: a ZooKeeper
|
||||
quorum, and the ZKFailoverController process (abbreviated as ZKFC).
|
||||
|
||||
Apache ZooKeeper is a highly available service for maintaining small amounts
|
||||
of coordination data, notifying clients of changes in that data, and
|
||||
monitoring clients for failures. The implementation of automatic HDFS failover
|
||||
relies on ZooKeeper for the following things:
|
||||
|
||||
* <<Failure detection>> - each of the NameNode machines in the cluster
|
||||
maintains a persistent session in ZooKeeper. If the machine crashes, the
|
||||
ZooKeeper session will expire, notifying the other NameNode that a failover
|
||||
should be triggered.
|
||||
|
||||
* <<Active NameNode election>> - ZooKeeper provides a simple mechanism to
|
||||
exclusively elect a node as active. If the current active NameNode crashes,
|
||||
another node may take a special exclusive lock in ZooKeeper indicating that
|
||||
it should become the next active.
|
||||
|
||||
The ZKFailoverController (ZKFC) is a new component which is a ZooKeeper client
|
||||
which also monitors and manages the state of the NameNode. Each of the
|
||||
machines which runs a NameNode also runs a ZKFC, and that ZKFC is responsible
|
||||
for:
|
||||
|
||||
* <<Health monitoring>> - the ZKFC pings its local NameNode on a periodic
|
||||
basis with a health-check command. So long as the NameNode responds in a
|
||||
timely fashion with a healthy status, the ZKFC considers the node
|
||||
healthy. If the node has crashed, frozen, or otherwise entered an unhealthy
|
||||
state, the health monitor will mark it as unhealthy.
|
||||
|
||||
* <<ZooKeeper session management>> - when the local NameNode is healthy, the
|
||||
ZKFC holds a session open in ZooKeeper. If the local NameNode is active, it
|
||||
also holds a special "lock" znode. This lock uses ZooKeeper's support for
|
||||
"ephemeral" nodes; if the session expires, the lock node will be
|
||||
automatically deleted.
|
||||
|
||||
* <<ZooKeeper-based election>> - if the local NameNode is healthy, and the
|
||||
ZKFC sees that no other node currently holds the lock znode, it will itself
|
||||
try to acquire the lock. If it succeeds, then it has "won the election", and
|
||||
is responsible for running a failover to make its local NameNode active. The
|
||||
failover process is similar to the manual failover described above: first,
|
||||
the previous active is fenced if necessary, and then the local NameNode
|
||||
transitions to active state.
|
||||
|
||||
For more details on the design of automatic failover, refer to the design
|
||||
document attached to HDFS-2185 on the Apache HDFS JIRA.
|
||||
|
||||
** Deploying ZooKeeper
|
||||
|
||||
In a typical deployment, ZooKeeper daemons are configured to run on three or
|
||||
five nodes. Since ZooKeeper itself has light resource requirements, it is
|
||||
acceptable to collocate the ZooKeeper nodes on the same hardware as the HDFS
|
||||
NameNode and Standby Node. Many operators choose to deploy the third ZooKeeper
|
||||
process on the same node as the YARN ResourceManager. It is advisable to
|
||||
configure the ZooKeeper nodes to store their data on separate disk drives from
|
||||
the HDFS metadata for best performance and isolation.
|
||||
|
||||
The setup of ZooKeeper is out of scope for this document. We will assume that
|
||||
you have set up a ZooKeeper cluster running on three or more nodes, and have
|
||||
verified its correct operation by connecting using the ZK CLI.
|
||||
|
||||
** Before you begin
|
||||
|
||||
Before you begin configuring automatic failover, you should shut down your
|
||||
cluster. It is not currently possible to transition from a manual failover
|
||||
setup to an automatic failover setup while the cluster is running.
|
||||
|
||||
** Configuring automatic failover
|
||||
|
||||
The configuration of automatic failover requires the addition of two new
|
||||
parameters to your configuration. In your <<<hdfs-site.xml>>> file, add:
|
||||
|
||||
----
|
||||
<property>
|
||||
<name>dfs.ha.automatic-failover.enabled</name>
|
||||
<value>true</value>
|
||||
</property>
|
||||
----
|
||||
|
||||
This specifies that the cluster should be set up for automatic failover.
|
||||
In your <<<core-site.xml>>> file, add:
|
||||
|
||||
----
|
||||
<property>
|
||||
<name>ha.zookeeper.quorum</name>
|
||||
<value>zk1.example.com:2181,zk2.example.com:2181,zk3.example.com:2181</value>
|
||||
</property>
|
||||
----
|
||||
|
||||
This lists the host-port pairs running the ZooKeeper service.
|
||||
|
||||
As with the parameters described earlier in the document, these settings may
|
||||
be configured on a per-nameservice basis by suffixing the configuration key
|
||||
with the nameservice ID. For example, in a cluster with federation enabled,
|
||||
you can explicitly enable automatic failover for only one of the nameservices
|
||||
by setting <<<dfs.ha.automatic-failover.enabled.my-nameservice-id>>>.
|
||||
|
||||
There are also several other configuration parameters which may be set to
|
||||
control the behavior of automatic failover; however, they are not necessary
|
||||
for most installations. Please refer to the configuration key specific
|
||||
documentation for details.
|
||||
|
||||
** Initializing HA state in ZooKeeper
|
||||
|
||||
After the configuration keys have been added, the next step is to initialize
|
||||
required state in ZooKeeper. You can do so by running the following command
|
||||
from one of the NameNode hosts.
|
||||
|
||||
----
|
||||
$ hdfs zkfc -formatZK
|
||||
----
|
||||
|
||||
This will create a znode in ZooKeeper inside of which the automatic failover
|
||||
system stores its data.
|
||||
|
||||
** Starting the cluster with <<<start-dfs.sh>>>
|
||||
|
||||
Since automatic failover has been enabled in the configuration, the
|
||||
<<<start-dfs.sh>>> script will now automatically start a ZKFC daemon on any
|
||||
machine that runs a NameNode. When the ZKFCs start, they will automatically
|
||||
select one of the NameNodes to become active.
|
||||
|
||||
** Starting the cluster manually
|
||||
|
||||
If you manually manage the services on your cluster, you will need to manually
|
||||
start the <<<zkfc>>> daemon on each of the machines that runs a NameNode. You
|
||||
can start the daemon by running:
|
||||
|
||||
----
|
||||
$ hadoop-daemon.sh start zkfc
|
||||
----
|
||||
|
||||
** Securing access to ZooKeeper
|
||||
|
||||
If you are running a secure cluster, you will likely want to ensure that the
|
||||
information stored in ZooKeeper is also secured. This prevents malicious
|
||||
clients from modifying the metadata in ZooKeeper or potentially triggering a
|
||||
false failover.
|
||||
|
||||
In order to secure the information in ZooKeeper, first add the following to
|
||||
your <<<core-site.xml>>> file:
|
||||
|
||||
----
|
||||
<property>
|
||||
<name>ha.zookeeper.auth</name>
|
||||
<value>@/path/to/zk-auth.txt</value>
|
||||
</property>
|
||||
<property>
|
||||
<name>ha.zookeeper.acl</name>
|
||||
<value>@/path/to/zk-acl.txt</value>
|
||||
</property>
|
||||
----
|
||||
|
||||
Please note the '@' character in these values -- this specifies that the
|
||||
configurations are not inline, but rather point to a file on disk.
|
||||
|
||||
The first configured file specifies a list of ZooKeeper authentications, in
|
||||
the same format as used by the ZK CLI. For example, you may specify something
|
||||
like:
|
||||
|
||||
----
|
||||
digest:hdfs-zkfcs:mypassword
|
||||
----
|
||||
...where <<<hdfs-zkfcs>>> is a unique username for ZooKeeper, and
|
||||
<<<mypassword>>> is some unique string used as a password.
|
||||
|
||||
Next, generate a ZooKeeper ACL that corresponds to this authentication, using
|
||||
a command like the following:
|
||||
|
||||
----
|
||||
$ java -cp $ZK_HOME/lib/*:$ZK_HOME/zookeeper-3.4.2.jar org.apache.zookeeper.server.auth.DigestAuthenticationProvider hdfs-zkfcs:mypassword
|
||||
output: hdfs-zkfcs:mypassword->hdfs-zkfcs:P/OQvnYyU/nF/mGYvB/xurX8dYs=
|
||||
----
|
||||
|
||||
Copy and paste the section of this output after the '->' string into the file
|
||||
<<<zk-acls.txt>>>, prefixed by the string "<<<digest:>>>". For example:
|
||||
|
||||
----
|
||||
digest:hdfs-zkfcs:vlUvLnd8MlacsE80rDuu6ONESbM=:rwcda
|
||||
----
|
||||
|
||||
In order for these ACLs to take effect, you should then rerun the
|
||||
<<<zkfc -formatZK>>> command as described above.
|
||||
|
||||
After doing so, you may verify the ACLs from the ZK CLI as follows:
|
||||
|
||||
----
|
||||
[zk: localhost:2181(CONNECTED) 1] getAcl /hadoop-ha
|
||||
'digest,'hdfs-zkfcs:vlUvLnd8MlacsE80rDuu6ONESbM=
|
||||
: cdrwa
|
||||
----
|
||||
|
||||
** Verifying automatic failover
|
||||
|
||||
Once automatic failover has been set up, you should test its operation. To do
|
||||
so, first locate the active NameNode. You can tell which node is active by
|
||||
visiting the NameNode web interfaces -- each node reports its HA state at the
|
||||
top of the page.
|
||||
|
||||
Once you have located your active NameNode, you may cause a failure on that
|
||||
node. For example, you can use <<<kill -9 <pid of NN>>>> to simulate a JVM
|
||||
crash. Or, you could power cycle the machine or unplug its network interface
|
||||
to simulate a different kind of outage. After triggering the outage you wish
|
||||
to test, the other NameNode should automatically become active within several
|
||||
seconds. The amount of time required to detect a failure and trigger a
|
||||
fail-over depends on the configuration of
|
||||
<<<ha.zookeeper.session-timeout.ms>>>, but defaults to 5 seconds.
|
||||
|
||||
If the test does not succeed, you may have a misconfiguration. Check the logs
|
||||
for the <<<zkfc>>> daemons as well as the NameNode daemons in order to further
|
||||
diagnose the issue.
|
||||
|
||||
|
||||
* Automatic Failover FAQ
|
||||
|
||||
* <<Is it important that I start the ZKFC and NameNode daemons in any
|
||||
particular order?>>
|
||||
|
||||
No. On any given node you may start the ZKFC before or after its corresponding
|
||||
NameNode.
|
||||
|
||||
* <<What additional monitoring should I put in place?>>
|
||||
|
||||
You should add monitoring on each host that runs a NameNode to ensure that the
|
||||
ZKFC remains running. In some types of ZooKeeper failures, for example, the
|
||||
ZKFC may unexpectedly exit, and should be restarted to ensure that the system
|
||||
is ready for automatic failover.
|
||||
|
||||
Additionally, you should monitor each of the servers in the ZooKeeper
|
||||
quorum. If ZooKeeper crashes, then automatic failover will not function.
|
||||
|
||||
* <<What happens if ZooKeeper goes down?>>
|
||||
|
||||
If the ZooKeeper cluster crashes, no automatic failovers will be triggered.
|
||||
However, HDFS will continue to run without any impact. When ZooKeeper is
|
||||
restarted, HDFS will reconnect with no issues.
|
||||
|
||||
* <<Can I designate one of my NameNodes as primary/preferred?>>
|
||||
|
||||
No. Currently, this is not supported. Whichever NameNode is started first will
|
||||
become active. You may choose to start the cluster in a specific order such
|
||||
that your preferred node starts first.
|
||||
|
||||
* <<How can I initiate a manual failover when automatic failover is
|
||||
configured?>>
|
||||
|
||||
Even if automatic failover is configured, you may initiate a manual failover
|
||||
using the same <<<hdfs haadmin>>> command. It will perform a coordinated
|
||||
failover.
|
||||
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Reference in New Issue
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