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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 Map Reduce Next Generation-${project.version} - Cluster Setup
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+ ---
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+ ---
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+ ${maven.build.timestamp}
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+
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+HDFS Federation
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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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+ This guide provides an overview of the HDFS Federation feature and
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+ how to configure and manage the federated cluster.
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+
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+* {Background}
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+
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+[./federation-background.gif] HDFS Layers
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+
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+ HDFS has two main layers:
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+
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+ * <<Namespace>>
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+
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+ * Consists of directories, files and blocks
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+
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+ * It supports all the namespace related file system operations such as
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+ create, delete, modify and list files and directories.
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+
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+ * <<Block Storage Service>> has two parts
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+
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+ * Block Management (which is done in Namenode)
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+
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+ * Provides datanode cluster membership by handling registrations, and
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+ periodic heart beats.
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+
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+ * Processes block reports and maintains location of blocks.
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+
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+ * Supports block related operations such as create, delete, modify and
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+ get block location.
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+
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+ * Manages replica placement and replication of a block for under
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+ replicated blocks and deletes blocks that are over replicated.
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+
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+ * Storage - is provided by datanodes by storing blocks on the local file
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+ system and allows read/write access.
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+
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+ The current HDFS architecture allows only a single namespace for the
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+ entire cluster. A single Namenode manages this namespace. HDFS
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+ Federation addresses limitation of current architecture by adding
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+ support multiple Namenodes/namespaces to HDFS file system.
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+
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+* {HDFS Federation}
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+
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+ In order to scale the name service horizontally, federation uses multiple
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+ independent Namenodes/namespaces. The Namenodes are federated, that is, the
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+ Namenodes are independent and don’t require coordination with each other.
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+ The datanodes are used as common storage for blocks by all the Namenodes.
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+ Each datanode registers with all the Namenodes in the cluster. Datanodes
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+ send periodic heartbeats and block reports and handles commands from the
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+ Namenodes.
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+
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+[./federation.gif] HDFS Federation Architecture
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+
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+
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+ <<Block Pool>>
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+
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+ A Block Pool is a set of blocks that belong to a single namespace.
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+ Datanodes store blocks for all the block pools in the cluster.
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+ It is managed independently of other block pools. This allows a namespace
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+ to generate Block IDs for new blocks without the need for coordination
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+ with the other namespaces. The failure of a Namenode does not prevent
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+ the datanode from serving other Namenodes in the cluster.
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+
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+ A Namespace and its block pool together are called Namespace Volume.
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+ It is a self-contained unit of management. When a Namenode/namespace
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+ is deleted, the corresponding block pool at the datanodes is deleted.
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+ Each namespace volume is upgraded as a unit, during cluster upgrade.
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+
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+ <<ClusterID>>
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+
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+ A new identifier <<ClusterID>> is added to identify all the nodes in
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+ the cluster. When a Namenode is formatted, this identifier is provided
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+ or auto generated. This ID should be used for formatting the other
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+ Namenodes into the cluster.
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+
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+** Key Benefits
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+
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+ * Namespace Scalability - HDFS cluster storage scales horizontally but
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+ the namespace does not. Large deployments or deployments using lot
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+ of small files benefit from scaling the namespace by adding more
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+ Namenodes to the cluster
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+
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+ * Performance - File system operation throughput is currently limited
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+ by a single Namenode. Adding more Namenodes to the cluster scales the
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+ file system read/write operations throughput.
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+
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+ * Isolation - A single Namenode offers no isolation in multi user
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+ environment. An experimental application can overload the Namenode
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+ and slow down production critical applications. With multiple Namenodes,
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+ different categories of applications and users can be isolated to
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+ different namespaces.
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+
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+* {Federation Configuration}
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+
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+ Federation configuration is <<backward compatible>> and allows existing
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+ single Namenode configuration to work without any change. The new
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+ configuration is designed such that all the nodes in the cluster have
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+ same configuration without the need for deploying different configuration
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+ based on the type of the node in the cluster.
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+
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+ A new abstraction called <<<NameServiceID>>> is added with
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+ federation. The Namenode and its corresponding secondary/backup/checkpointer
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+ nodes belong to this. To support single configuration file, the Namenode and
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+ secondary/backup/checkpointer configuration parameters are suffixed with
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+ <<<NameServiceID>>> and are added to the same configuration file.
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+
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+
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+** Configuration:
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+
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+ <<Step 1>>: Add the following parameters to your configuration:
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+ <<<dfs.federation.nameservices>>>: Configure with list of comma separated
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+ NameServiceIDs. This will be used by Datanodes to determine all the
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+ Namenodes in the cluster.
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+
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+ <<Step 2>>: For each Namenode and Secondary Namenode/BackupNode/Checkpointer
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+ add the following configuration suffixed with the corresponding
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+ <<<NameServiceID>>> into the common configuration file.
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+
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+*---------------------+--------------------------------------------+
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+|| Daemon || Configuration Parameter |
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+*---------------------+--------------------------------------------+
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+| Namenode | <<<dfs.namenode.rpc-address>>> |
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+| | <<<dfs.namenode.servicerpc-address>>> |
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+| | <<<dfs.namenode.http-address>>> |
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+| | <<<dfs.namenode.https-address>>> |
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+| | <<<dfs.namenode.keytab.file>>> |
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+| | <<<dfs.namenode.name.dir>>> |
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+| | <<<dfs.namenode.edits.dir>>> |
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+| | <<<dfs.namenode.checkpoint.dir>>> |
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+| | <<<dfs.namenode.checkpoint.edits.dir>>> |
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+*---------------------+--------------------------------------------+
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+| Secondary Namenode | <<<dfs.namenode.secondary.http-address>>> |
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+| | <<<dfs.secondary.namenode.keytab.file>>> |
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+*---------------------+--------------------------------------------+
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+| BackupNode | <<<dfs.namenode.backup.address>>> |
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+| | <<<dfs.secondary.namenode.keytab.file>>> |
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+*---------------------+--------------------------------------------+
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+
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+ Here is an example configuration with two namenodes:
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+
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+----
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+<configuration>
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+ <property>
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+ <name>dfs.federation.nameservices</name>
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+ <value>ns1,ns2</value>
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+ </property>
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+ <property>
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+ <name>dfs.namenode.rpc-address.ns1</name>
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+ <value>hdfs://nn-host1:rpc-port</value>
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+ </property>
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+ <property>
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+ <name>dfs.namenode.http-address.ns1</name>
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+ <value>nn-host1:http-port</value>
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+ </property>
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+ <property>
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+ <name>dfs.namenode.secondaryhttp-address.ns1</name>
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+ <value>snn-host1:http-port</value>
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+ </property>
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+ <property>
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+ <name>dfs.namenode.rpc-address.ns2</name>
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+ <value>hdfs://nn-host2:rpc-port</value>
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+ </property>
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+ <property>
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+ <name>dfs.namenode.http-address.ns2</name>
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+ <value>nn-host2:http-port</value>
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+ </property>
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+ <property>
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+ <name>dfs.namenode.secondaryhttp-address.ns2</name>
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+ <value>snn-host2:http-port</value>
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+ </property>
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+
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+ .... Other common configuration ...
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+</configuration>
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+----
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+
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+** Formatting Namenodes
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+
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+ <<Step 1>>: Format a namenode using the following command:
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+
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+----
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+> $HADOOP_PREFIX_HOME/bin/hdfs namenode -format [-clusterId <cluster_id>]
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+----
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+ Choose a unique cluster_id, which will not conflict other clusters in
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+ your environment. If it is not provided, then a unique ClusterID is
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+ auto generated.
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+
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+ <<Step 2>>: Format additional namenode using the following command:
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+
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+----
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+> $HADOOP_PREFIX_HOME/bin/hdfs namenode -format -clusterId <cluster_id>
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+----
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+ Note that the cluster_id in step 2 must be same as that of the
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+ cluster_id in step 1. If they are different, the additional Namenodes
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+ will not be part of the federated cluster.
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+
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+** Upgrading from older release to 0.23 and configuring federation
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+
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+ Older releases supported a single Namenode. Here are the steps enable
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+ federation:
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+
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+ Step 1: Upgrade the cluster to newer release. During upgrade you can
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+ provide a ClusterID as follows:
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+
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+----
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+> $HADOOP_PREFIX_HOME/bin/hdfs start namenode --config $HADOOP_CONF_DIR -upgrade -clusterId <cluster_ID>
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+----
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+ If ClusterID is not provided, it is auto generated.
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+
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+** Adding a new Namenode to an existing HDFS cluster
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+
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+ Follow the following steps:
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+
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+ * Add configuration parameter <<<dfs.federation.nameservices>>> to
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+ the configuration.
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+
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+ * Update the configuration with NameServiceID suffix. Configuration
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+ key names have changed post release 0.20. You must use new configuration
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+ parameter names, for federation.
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+
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+ * Add new Namenode related config to the configuration files.
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+
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+ * Propagate the configuration file to the all the nodes in the cluster.
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+
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+ * Start the new Namenode, Secondary/Backup.
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+
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+ * Refresh the datanodes to pickup the newly added Namenode by running
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+ the following command:
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+
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+----
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+> $HADOOP_PREFIX_HOME/bin/hdfs dfadmin -refreshNameNode <datanode_host_name>:<datanode_rpc_port>
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+----
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+
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+ * The above command must be run against all the datanodes in the cluster.
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+
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+* {Managing the cluster}
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+
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+** Starting and stopping cluster
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+
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+ To start the cluster run the following command:
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+
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+----
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+> $HADOOP_PREFIX_HOME/bin/start-dfs.sh
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+----
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+
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+ To start the cluster run the following command:
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+
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+----
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+> $HADOOP_PREFIX_HOME/bin/stop-dfs.sh
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+----
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+
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+ These commands can be run from any node where the HDFS configuration is
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+ available. The command uses configuration to determine the Namenodes
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+ in the cluster and starts the Namenode process on those nodes. The
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+ datanodes are started on nodes specified in the <<<slaves>>> file. The
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+ script can be used as reference for building your own scripts for
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+ starting and stopping the cluster.
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+
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+** Balancer
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+
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+ Balancer has been changed to work with multiple Namenodes in the cluster to
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+ balance the cluster. Balancer can be run using the command:
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+
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+----
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+"$HADOOP_PREFIX"/bin/hadoop-daemon.sh --config $HADOOP_CONF_DIR --script "$bin"/hdfs start balancer [-policy <policy>]
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+----
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+
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+ Policy could be:
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+
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+ * <<<node>>> - this is the <default> policy. This balances the storage at
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+ the datanode level. This is similar to balancing policy from prior releases.
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+
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+ * <<<blockpool>>> - this balances the storage at the block pool level.
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+ Balancing at block pool level balances storage at the datanode level also.
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+
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+ Note that Balander only balances the data and does not balance the namespace.
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+
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+** Decommissioning
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+
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+ Decommissioning is similar to prior releases. The nodes that need to be
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+ decommissioning are added to the exclude file at all the Namenode. Each
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+ Namenode decommissions its Block Pool. When all the Namenodes finish
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+ decommissioning a datanode, the datanode is considered to be decommissioned.
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+
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+ <<Step 1>>: To distributed an exclude file to all the Namenodes, use the
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+ following command:
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+
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+----
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+"$HADOOP_PREFIX"/bin/distributed-exclude.sh <exclude_file>
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+----
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+
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+ <<Step 2>>: Refresh all the Namenodes to pick up the new exclude file.
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+
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+----
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+"$HADOOP_PREFIX"/bin/refresh-namenodes.sh
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+----
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+
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+ The above command uses HDFS configuration to determine the Namenodes
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+ configured in the cluster and refreshes all the Namenodes to pick up
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+ the new exclude file.
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+
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+** Cluster Web Console
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+
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+ Similar to Namenode status web page, a Cluster Web Console is added in
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+ federation to monitor the federated cluster at
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+ <<<http://<any_nn_host:port>/dfsclusterhealth.jsp>>>.
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+ Any Namenode in the cluster can be used to access this web page.
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+
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+ The web page provides the following information:
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+
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+ * Cluster summary that shows number of files, number of blocks and
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+ total configured storage capacity, available and used storage information
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+ for the entire cluster.
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+
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+ * Provides list of Namenodes and summary that includes number of files,
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+ blocks, missing blocks, number of live and dead data nodes for each
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+ Namenode. It also provides a link to conveniently access Namenode web UI.
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+
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+ * It also provides decommissioning status of datanodes.
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+
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+
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Suresh Srinivas
