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| |
| --- |
| Hadoop Distributed File System-${project.version} - High Availability |
| --- |
| --- |
| ${maven.build.timestamp} |
| |
| HDFS High Availability |
| |
| \[ {{{./index.html}Go Back}} \] |
| |
| %{toc|section=1|fromDepth=0} |
| |
| * {Purpose} |
| |
| This guide provides an overview of the HDFS High Availability (HA) feature and |
| how to configure and manage an HA HDFS cluster. |
| |
| This document assumes that the reader has a general understanding of |
| general components and node types in an HDFS cluster. Please refer to the |
| HDFS Architecture guide for details. |
| |
| * {Background} |
| |
| Prior to Hadoop 0.23.2, the NameNode was a single point of failure (SPOF) in |
| an HDFS cluster. Each cluster had a single NameNode, and if that machine or |
| process became unavailable, the cluster as a whole would be unavailable |
| until the NameNode was either restarted or brought up on a separate machine. |
| |
| This impacted the total availability of the HDFS cluster in two major ways: |
| |
| * In the case of an unplanned event such as a machine crash, the cluster would |
| be unavailable until an operator restarted the NameNode. |
| |
| * Planned maintenance events such as software or hardware upgrades on the |
| NameNode machine would result in windows of cluster downtime. |
| |
| The HDFS High Availability feature addresses the above problems by providing |
| the option of running two redundant NameNodes in the same cluster in an |
| Active/Passive configuration with a hot standby. This allows a fast failover to |
| a new NameNode in the case that a machine crashes, or a graceful |
| administrator-initiated failover for the purpose of planned maintenance. |
| |
| * {Architecture} |
| |
| In a typical HA cluster, two separate machines are configured as NameNodes. |
| At any point in time, exactly one of the NameNodes is in an <Active> state, |
| and the other is in a <Standby> state. The Active NameNode is responsible |
| for all client operations in the cluster, while the Standby is simply acting |
| as a slave, maintaining enough state to provide a fast failover if |
| necessary. |
| |
| In order for the Standby node to keep its state synchronized with the Active |
| node, the current implementation requires that the two nodes both have access |
| to a directory on a shared storage device (eg an NFS mount from a NAS). This |
| restriction will likely be relaxed in future versions. |
| |
| When any namespace modification is performed by the Active node, it durably |
| logs a record of the modification to an edit log file stored in the shared |
| directory. The Standby node is constantly watching this directory for edits, |
| and as it sees the edits, it applies them to its own namespace. In the event of |
| a failover, the Standby will ensure that it has read all of the edits from the |
| shared storage before promoting itself to the Active state. This ensures that |
| the namespace state is fully synchronized before a failover occurs. |
| |
| In order to provide a fast failover, it is also necessary that the Standby node |
| have up-to-date information regarding the location of blocks in the cluster. |
| In order to achieve this, the DataNodes are configured with the location of |
| both NameNodes, and send block location information and heartbeats to both. |
| |
| It is vital for the correct operation of an HA cluster that only one of the |
| NameNodes be Active at a time. Otherwise, the namespace state would quickly |
| diverge between the two, risking data loss or other incorrect results. In |
| order to ensure this property and prevent the so-called "split-brain scenario," |
| the administrator must configure at least one <fencing method> for the shared |
| storage. During a failover, if it cannot be verified that the previous Active |
| node has relinquished its Active state, the fencing process is responsible for |
| cutting off the previous Active's access to the shared edits storage. This |
| prevents it from making any further edits to the namespace, allowing the new |
| Active to safely proceed with failover. |
| |
| <<Note:>> Currently, only manual failover is supported. This means the HA |
| NameNodes are incapable of automatically detecting a failure of the Active |
| NameNode, and instead rely on the operator to manually initiate a failover. |
| Automatic failure detection and initiation of a failover will be implemented in |
| future versions. |
| |
| * {Hardware resources} |
| |
| In order to deploy an HA cluster, you should prepare the following: |
| |
| * <<NameNode machines>> - the machines on which you run the Active and |
| Standby NameNodes should have equivalent hardware to each other, and |
| equivalent hardware to what would be used in a non-HA cluster. |
| |
| * <<Shared storage>> - you will need to have a shared directory which both |
| NameNode machines can have read/write access to. Typically this is a remote |
| filer which supports NFS and is mounted on each of the NameNode machines. |
| Currently only a single shared edits directory is supported. Thus, the |
| availability of the system is limited by the availability of this shared edits |
| directory, and therefore in order to remove all single points of failure there |
| needs to be redundancy for the shared edits directory. Specifically, multiple |
| network paths to the storage, and redundancy in the storage itself (disk, |
| network, and power). Beacuse of this, it is recommended that the shared storage |
| server be a high-quality dedicated NAS appliance rather than a simple Linux |
| server. |
| |
| Note that, in an HA cluster, the Standby NameNode also performs checkpoints of |
| the namespace state, and thus it is not necessary to run a Secondary NameNode, |
| CheckpointNode, or BackupNode in an HA cluster. In fact, to do so would be an |
| error. This also allows one who is reconfiguring a non-HA-enabled HDFS cluster |
| to be HA-enabled to reuse the hardware which they had previously dedicated to |
| the Secondary NameNode. |
| |
| * {Deployment} |
| |
| ** Configuration overview |
| |
| Similar to Federation configuration, HA configuration is backward compatible |
| and allows existing single NameNode configurations to work without change. |
| The new configuration is designed such that all the nodes in the cluster may |
| have the same configuration without the need for deploying different |
| configuration files to different machines based on the type of the node. |
| |
| Like HDFS Federation, HA clusters reuse the <<<nameservice ID>>> to identify a |
| single HDFS instance that may in fact consist of multiple HA NameNodes. In |
| addition, a new abstraction called <<<NameNode ID>>> is added with HA. Each |
| distinct NameNode in the cluster has a different NameNode ID to distinguish it. |
| To support a single configuration file for all of the NameNodes, the relevant |
| configuration parameters are suffixed with the <<nameservice ID>> as well as |
| the <<NameNode ID>>. |
| |
| ** Configuration details |
| |
| To configure HA NameNodes, you must add several configuration options to your |
| <<hdfs-site.xml>> configuration file. |
| |
| The order in which you set these configurations is unimportant, but the values |
| you choose for <<dfs.federation.nameservices>> and |
| <<dfs.ha.namenodes.[nameservice ID]>> will determine the keys of those that |
| follow. Thus, you should decide on these values before setting the rest of the |
| configuration options. |
| |
| * <<dfs.federation.nameservices>> - the logical name for this new nameservice |
| |
| Choose a logical name for this nameservice, for example "mycluster", and use |
| this logical name for the value of this config option. The name you choose is |
| arbitrary. It will be used both for configuration and as the authority |
| component of absolute HDFS paths in the cluster. |
| |
| <<Note:>> If you are also using HDFS Federation, this configuration setting |
| should also include the list of other nameservices, HA or otherwise, as a |
| comma-separated list. |
| |
| ---- |
| <property> |
| <name>dfs.federation.nameservices</name> |
| <value>mycluster</value> |
| </property> |
| ---- |
| |
| * <<dfs.ha.namenodes.[nameservice ID]>> - unique identifiers for each NameNode in the nameservice |
| |
| Configure with a list of comma-separated NameNode IDs. This will be used by |
| DataNodes to determine all the NameNodes in the cluster. For example, if you |
| used "mycluster" as the nameservice ID previously, and you wanted to use "nn1" |
| and "nn2" as the individual IDs of the NameNodes, you would configure this as |
| such: |
| |
| ---- |
| <property> |
| <name>dfs.ha.namenodes.mycluster</name> |
| <value>nn1,nn2</value> |
| </property> |
| ---- |
| |
| <<Note:>> Currently, only a maximum of two NameNodes may be configured per |
| nameservice. |
| |
| * <<dfs.namenode.rpc-address.[nameservice ID].[name node ID]>> - the fully-qualified RPC address for each NameNode to listen on |
| |
| For both of the previously-configured NameNode IDs, set the full address and |
| IPC port of the NameNode processs. Note that this results in two separate |
| configuration options. For example: |
| |
| ---- |
| <property> |
| <name>dfs.namenode.rpc-address.mycluster.nn1</name> |
| <value>machine1.example.com:8020</value> |
| </property> |
| <property> |
| <name>dfs.namenode.rpc-address.mycluster.nn2</name> |
| <value>machine2.example.com:8020</value> |
| </property> |
| ---- |
| |
| <<Note:>> You may similarly configure the "<<servicerpc-address>>" setting if |
| you so desire. |
| |
| * <<dfs.namenode.http-address.[nameservice ID].[name node ID]>> - the fully-qualified HTTP address for each NameNode to listen on |
| |
| Similarly to <rpc-address> above, set the addresses for both NameNodes' HTTP |
| servers to listen on. For example: |
| |
| ---- |
| <property> |
| <name>dfs.namenode.http-address.mycluster.nn1</name> |
| <value>machine1.example.com:50070</value> |
| </property> |
| <property> |
| <name>dfs.namenode.http-address.mycluster.nn2</name> |
| <value>machine2.example.com:50070</value> |
| </property> |
| ---- |
| |
| <<Note:>> If you have Hadoop's security features enabled, you should also set |
| the <https-address> similarly for each NameNode. |
| |
| * <<dfs.namenode.shared.edits.dir>> - the location of the shared storage directory |
| |
| This is where one configures the path to the remote shared edits directory |
| which the Standby NameNode uses to stay up-to-date with all the file system |
| changes the Active NameNode makes. <<You should only configure one of these |
| directories.>> This directory should be mounted r/w on both NameNode machines. |
| The value of this setting should be the absolute path to this directory on the |
| NameNode machines. For example: |
| |
| ---- |
| <property> |
| <name>dfs.namenode.shared.edits.dir</name> |
| <value>file:///mnt/filer1/dfs/ha-name-dir-shared</value> |
| </property> |
| ---- |
| |
| * <<dfs.client.failover.proxy.provider.[nameservice ID]>> - the Java class that HDFS clients use to contact the Active NameNode |
| |
| 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> |
| </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 critical for correctness of the system that only one NameNode be in the |
| Active state at any given time. Thus, during a failover, we first ensure that |
| the Active NameNode is either in the Standby state, or the process has |
| terminated, before transitioning the other NameNode to the Active state. In |
| order to do this, you must configure at least one <<fencing method.>> These 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> |
| </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> |
| --- |
| |
| ** Deployment details |
| |
| After all of the necessary configuration options have been set, 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 using <scp> or a similar utility. The location |
| of the directories containing the NameNode metadata are configured via the |
| configuration options <<dfs.namenode.name.dir>> and/or |
| <<dfs.namenode.edits.dir>>. At this time, you should also ensure that the |
| shared edits dir (as configured by <<dfs.namenode.shared.edits.dir>>) includes |
| all recent edits files which are in your NameNode metadata 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. |