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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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+ ---
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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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+
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+HDFS High Availability
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+
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+ \[ {{{./index.html}Go Back}} \]
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+
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+%{toc|section=1|fromDepth=0}
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+
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+* {Purpose}
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+
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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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+
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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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+
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+* {Background}
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+
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+ Prior to Hadoop 0.23.2, 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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+
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+ This impacted the total availability of the HDFS cluster in two major ways:
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+
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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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+
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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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+
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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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+
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+* {Architecture}
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+
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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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+
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+ In order for the Standby node to keep its state synchronized with the Active
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+ node, the current implementation requires that the two nodes both have access
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+ to a directory on a shared storage device (eg an NFS mount from a NAS). This
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+ restriction will likely be relaxed in future versions.
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+
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+ When any namespace modification is performed by the Active node, it durably
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+ logs a record of the modification to an edit log file stored in the shared
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+ directory. The Standby node is constantly watching this directory for edits,
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+ and as it sees the edits, it applies them to its own namespace. In the event of
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+ a failover, the Standby will ensure that it has read all of the edits from the
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+ shared storage before promoting itself to the Active state. This ensures that
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+ the namespace state is fully synchronized before a failover occurs.
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+
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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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+
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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 administrator must configure at least one <fencing method> for the shared
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+ storage. During a failover, if it cannot be verified that the previous Active
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+ node has relinquished its Active state, the fencing process is responsible for
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+ cutting off the previous Active's access to the shared edits storage. This
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+ prevents it from making any further edits to the namespace, allowing the new
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+ Active to safely proceed with failover.
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+
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+ <<Note:>> Currently, only manual failover is supported. This means the HA
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+ NameNodes are incapable of automatically detecting a failure of the Active
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+ NameNode, and instead rely on the operator to manually initiate a failover.
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+ Automatic failure detection and initiation of a failover will be implemented in
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+ future versions.
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+
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+* {Hardware resources}
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+
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+ In order to deploy an HA cluster, you should prepare the following:
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+
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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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+
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+ * <<Shared storage>> - you will need to have a shared directory which both
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+ NameNode machines can have read/write access to. Typically this is a remote
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+ filer which supports NFS and is mounted on each of the NameNode machines.
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+ Currently only a single shared edits directory is supported. Thus, the
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+ availability of the system is limited by the availability of this shared edits
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+ directory, and therefore in order to remove all single points of failure there
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+ needs to be redundancy for the shared edits directory. Specifically, multiple
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+ network paths to the storage, and redundancy in the storage itself (disk,
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+ network, and power). Beacuse of this, it is recommended that the shared storage
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+ server be a high-quality dedicated NAS appliance rather than a simple Linux
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+ server.
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+
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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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+
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+* {Deployment}
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+
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+** Configuration overview
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+
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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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+
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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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+
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+** Configuration details
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+
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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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+
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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.federation.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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+
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+ * <<dfs.federation.nameservices>> - the logical name for this new nameservice
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+
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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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+
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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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+----
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+<property>
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+ <name>dfs.federation.nameservices</name>
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+ <value>mycluster</value>
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+</property>
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+----
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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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+
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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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+----
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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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+
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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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+
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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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+
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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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+----
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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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+
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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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+
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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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+
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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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+----
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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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+
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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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+
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+ * <<dfs.namenode.shared.edits.dir>> - the location of the shared storage directory
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+
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+ This is where one configures the path to the remote shared edits directory
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+ which the Standby NameNode uses to stay up-to-date with all the file system
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+ changes the Active NameNode makes. <<You should only configure one of these
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+ directories.>> This directory should be mounted r/w on both NameNode machines.
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+ The value of this setting should be the absolute path to this directory on the
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+ NameNode machines. For example:
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+
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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>file:///mnt/filer1/dfs/ha-name-dir-shared</value>
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+</property>
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+----
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+
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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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+
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+ Configure the name of the Java class which will be used by the DFS Client to
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+ determine which NameNode is the current Active, and therefore which NameNode is
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+ currently serving client requests. The only implementation which currently
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+ ships with Hadoop is the <<ConfiguredFailoverProxyProvider>>, so use this
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+ unless you are using a custom one. For example:
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+
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+----
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+<property>
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+ <name>dfs.client.failover.proxy.provider.mycluster</name>
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+ <value>org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider</value>
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+</property>
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+----
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+
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+ * <<dfs.ha.fencing.methods>> - a list of scripts or Java classes which will be used to fence the Active NameNode during a failover
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+
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+ It is critical for correctness of the system that only one NameNode be in the
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+ Active state at any given time. Thus, during a failover, we first ensure that
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+ the Active NameNode is either in the Standby state, or the process has
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+ terminated, before transitioning the other NameNode to the Active state. In
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+ order to do this, you must configure at least one <<fencing method.>> These are
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+ configured as a carriage-return-separated list, which will be attempted in order
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+ until one indicates that fencing has succeeded. There are two methods which
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+ ship with Hadoop: <shell> and <sshfence>. For information on implementing
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+ your own custom fencing method, see the <org.apache.hadoop.ha.NodeFencer> class.
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+
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+ * <<sshfence>> - SSH to the Active NameNode and kill the process
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+
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+ The <sshfence> option SSHes to the target node and uses <fuser> to kill the
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+ process listening on the service's TCP port. In order for this fencing option
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+ to work, it must be able to SSH to the target node without providing a
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+ passphrase. Thus, one must also configure the
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+ <<dfs.ha.fencing.ssh.private-key-files>> option, which is a
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+ comma-separated list of SSH private key files. For example:
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+
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+---
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+<property>
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+ <name>dfs.ha.fencing.methods</name>
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+ <value>sshfence</value>
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+</property>
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+
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+<property>
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+ <name>dfs.ha.fencing.ssh.private-key-files</name>
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+ <value>/home/exampleuser/.ssh/id_rsa</value>
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+</property>
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+---
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+
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+ Optionally, one may configure a non-standard username or port to perform the
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+ SSH. One may also configure a timeout, in milliseconds, for the SSH, after
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+ which this fencing method will be considered to have failed. It may be
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+ configured like so:
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+
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+---
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+<property>
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+ <name>dfs.ha.fencing.methods</name>
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+ <value>sshfence([[username][:port]])</value>
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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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+</property>
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+---
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+
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+ * <<shell>> - run an arbitrary shell command to fence the Active NameNode
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+
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+ The <shell> fencing method runs an arbitrary shell command. It may be
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+ configured like so:
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+
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+---
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+<property>
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+ <name>dfs.ha.fencing.methods</name>
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+ <value>shell(/path/to/my/script.sh arg1 arg2 ...)</value>
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+</property>
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+---
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+
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+ The string between '(' and ')' is passed directly to a bash shell and may not
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+ include any closing parentheses.
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+
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+ When executed, the first argument to the configured script will be the address
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+ of the NameNode to be fenced, followed by all arguments specified in the
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+ configuration.
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+
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+ The shell command will be run with an environment set up to contain all of the
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+ current Hadoop configuration variables, with the '_' character replacing any
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+ '.' characters in the configuration keys. If the shell command returns an exit
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+ code of 0, the fencing is determined to be successful. If it returns any other
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+ exit code, the fencing was not successful and the next fencing method in the
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+ list will be attempted.
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+
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+ <<Note:>> This fencing method does not implement any timeout. If timeouts are
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+ necessary, they should be implemented in the shell script itself (eg by forking
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+ a subshell to kill its parent in some number of seconds).
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+
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+ * <<fs.defaultFS>> - the default path prefix used by the Hadoop FS client when none is given
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+
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+ Optionally, you may now configure the default path for Hadoop clients to use
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+ the new HA-enabled logical URI. If you used "mycluster" as the nameservice ID
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+ earlier, this will be the value of the authority portion of all of your HDFS
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+ paths. This may be configured like so, in your <<core-site.xml>> file:
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+
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+---
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+<property>
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+ <name>fs.defaultFS</name>
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+ <value>hdfs://mycluster</value>
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+</property>
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+---
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+
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+** Deployment details
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+
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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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+
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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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+
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+ You can visit each of the NameNodes' web pages separately by browsing to their
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+ configured HTTP addresses. You should notice that next to the configured
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+ address will be the HA state of the NameNode (either "standby" or "active".)
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+ Whenever an HA NameNode starts, it is initially in the Standby state.
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+
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+** Administrative commands
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+
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+ Now that your HA NameNodes are configured and started, you will have access
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+ to some additional commands to administer your HA HDFS cluster. Specifically,
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+ you should familiarize yourself with all of the subcommands of the "<hdfs
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+ haadmin>" command. Running this command without any additional arguments will
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+ display the following usage information:
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+
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+---
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+Usage: DFSHAAdmin [-ns <nameserviceId>]
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+ [-transitionToActive <serviceId>]
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+ [-transitionToStandby <serviceId>]
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+ [-failover [--forcefence] [--forceactive] <serviceId> <serviceId>]
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+ [-getServiceState <serviceId>]
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+ [-checkHealth <serviceId>]
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+ [-help <command>]
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+---
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+
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+ This guide describes high-level uses of each of these subcommands. For
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+ specific usage information of each subcommand, you should run "<hdfs haadmin
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+ -help <command>>".
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+
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+ * <<transitionToActive>> and <<transitionToStandby>> - transition the state of the given NameNode to Active or Standby
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+
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+ These subcommands cause a given NameNode to transition to the Active or Standby
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+ state, respectively. <<These commands do not attempt to perform any fencing,
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+ and thus should rarely be used.>> Instead, one should almost always prefer to
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+ use the "<hdfs haadmin -failover>" subcommand.
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+
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+ * <<failover>> - initiate a failover between two NameNodes
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+
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+ This subcommand causes a failover from the first provided NameNode to the
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+ second. If the first NameNode is in the Standby state, this command simply
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+ transitions the second to the Active state without error. If the first NameNode
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+ is in the Active state, an attempt will be made to gracefully transition it to
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+ the Standby state. If this fails, the fencing methods (as configured by
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+ <<dfs.ha.fencing.methods>>) will be attempted in order until one
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+ succeeds. Only after this process will the second NameNode be transitioned to
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+ the Active state. If no fencing method succeeds, the second NameNode will not
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+ be transitioned to the Active state, and an error will be returned.
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+
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+ * <<getServiceState>> - determine whether the given NameNode is Active or Standby
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+
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+ Connect to the provided NameNode to determine its current state, printing
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+ either "standby" or "active" to STDOUT appropriately. This subcommand might be
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+ used by cron jobs or monitoring scripts which need to behave differently based
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+ on whether the NameNode is currently Active or Standby.
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+
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+ * <<checkHealth>> - check the health of the given NameNode
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+
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+ Connect to the provided NameNode to check its health. The NameNode is capable
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+ of performing some diagnostics on itself, including checking if internal
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+ services are running as expected. This command will return 0 if the NameNode is
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+ healthy, non-zero otherwise. One might use this command for monitoring
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+ purposes.
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+
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+ <<Note:>> This is not yet implemented, and at present will always return
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+ success, unless the given NameNode is completely down.
|
Aaron Myers