hadoop配置名称节点HA原理
ArchitectureIn a typical HA clusiter, 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, both nodes communicate with a group of separate daemons called “JournalNodes” (JNs). When any namespace modification is performed by the Active node, it durably logs a record of the modification to a majority of these JNs. The Standby node is capable of reading the edits from the JNs, and is constantly watching them for changes to the edit log. As the Standby Node 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 JounalNodes 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 JournalNodes will only ever allow a single NameNode to be a writer at a time. During a failover, the NameNode which is to become active will simply take over the role of writing to the JournalNodes, which will effectively prevent the other NameNode from continuing in the Active state, allowing the new Active to safely proceed with failover.
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.
[*]
JournalNode machines - the machines on which you run the JournalNodes. The JournalNode daemon is>
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
hdfs-site.xml
dfs.nameservices
mycluster
dfs.ha.namenodes.mycluster
nn1,nn2
dfs.namenode.rpc-address.mycluster.nn1
192.168.153.201:8020
dfs.namenode.rpc-address.mycluster.nn2
192.168.153.205:8020
dfs.namenode.http-address.mycluster.nn1
192.168.153.201:50070
dfs.namenode.http-address.mycluster.nn2
192.168.153.205:50070
dfs.namenode.shared.edits.dir
qjournal://192.168.153.202:8485;192.168.153.203:8485;192.168.153.204:8485/mycluster
dfs.client.failover.proxy.provider.mycluster
org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider
dfs.ha.fencing.methods
sshfence
dfs.ha.fencing.ssh.private-key-files
/home/centos/.ssh/id_rsa
dfs.journalnode.edits.dir
/home/centos/hadoop/hdfs/journal
core-site.xml
fs.defaultFS
hdfs://mycluster
Notes
1、Currently, only a maximum of two NameNodes may be configured per nameservice.
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