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R3704/hadoop-2.3.0-src/hadoop-hdfs-project/hadoop-hdfs/src/contrib/bkjournal/README.txt
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This module provides a BookKeeper backend for HFDS Namenode write
ahead logging.
BookKeeper is a highly available distributed write ahead logging
system. For more details, see
http://zookeeper.apache.org/bookkeeper
-------------------------------------------------------------------------------
How do I build?
To generate the distribution packages for BK journal, do the
following.
$ mvn clean package -Pdist
This will generate a jar with all the dependencies needed by the journal
manager,
target/hadoop-hdfs-bkjournal-<VERSION>.jar
Note that the -Pdist part of the build command is important, as otherwise
the dependencies would not be packaged in the jar.
-------------------------------------------------------------------------------
How do I use the BookKeeper Journal?
To run a HDFS namenode using BookKeeper as a backend, copy the bkjournal
jar, generated above, into the lib directory of hdfs. In the standard
distribution of HDFS, this is at $HADOOP_HDFS_HOME/share/hadoop/hdfs/lib/
cp target/hadoop-hdfs-bkjournal-<VERSION>.jar \
$HADOOP_HDFS_HOME/share/hadoop/hdfs/lib/
Then, in hdfs-site.xml, set the following properties.
<property>
<name>dfs.namenode.edits.dir</name>
<value>bookkeeper://localhost:2181/bkjournal,file:///path/for/edits</value>
</property>
<property>
<name>dfs.namenode.edits.journal-plugin.bookkeeper</name>
<value>org.apache.hadoop.contrib.bkjournal.BookKeeperJournalManager</value>
</property>
In this example, the namenode is configured to use 2 write ahead
logging devices. One writes to BookKeeper and the other to a local
file system. At the moment is is not possible to only write to
BookKeeper, as the resource checker explicitly checked for local
disks currently.
The given example, configures the namenode to look for the journal
metadata at the path /bkjournal on the a standalone zookeeper ensemble
at localhost:2181. To configure a multiple host zookeeper ensemble,
separate the hosts with semicolons. For example, if you have 3
zookeeper servers, zk1, zk2 & zk3, each listening on port 2181, you
would specify this with
bookkeeper://zk1:2181;zk2:2181;zk3:2181/bkjournal
The final part /bkjournal specifies the znode in zookeeper where
ledger metadata will be store. Administrators can set this to anything
they wish.
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