DataStax Enterprise 4.0

About the Cassandra File System

A Hive or Pig analytics job requires a Hadoop file system to function. DataStax Enterprise provides a replacement for the Hadoop Distributed File System (HDFS) called the Cassandra File System (CassandraFS), which serves this purpose. When an analytics node starts up, DataStax Enterprise creates a default CassandraFS rooted at cfs:/ and an archive file system named cfs-archive.

Configuring a CFS superuser

A CFS superuser is the DSE daemon user, the user who starts DataStax Enterprise. A cassandra superuser, set up using the CQL CREATE USER command, is also a CFS superuser.

A CFS superuser can modify files in the CassandraFS without any restrictions. Files that a superuser adds to the CassandraFS are password-protected.

Deleting files from the Cassandra File System

Cassandra does not immediately remove deleted data from disk when you use the dse hadoop fs -rm file command. Instead, Cassandra treats the deleted data like any data deleted from Cassandra. A tombstone is written to indicate the new data status. Data marked with a tombstone exist for a configured time period (defined by the gc_grace_seconds value set on the table). When the grace period expires, the compaction process permanently deletes the data. You do not have to manually remove expired data.

Using multiple Cassandra File Systems

DataStax Enterprise supports multiple CassandraFS's. Some typical reasons for using an additional CassandraFS are:
  • To isolate hadoop-related jobs
  • To configure keyspace replication by job
  • To segregate file systems in different physical data centers
  • To separate Hadoop data in some other way

Creating multiple Cassandra File Systems

Procedure

  1. Open the core-site.xml file for editing. The location of the file depends on your installation:
    • Tarball installs: /etc/dse/hadoop
    • Packaged installation: install_location/resources/hadoop/conf
  2. Add one or more property elements to core-site.xml using this format:
    <property>
      <name>fs.cfs-<filesystemname.impl></name>
      <value>com.datastax.bdp.hadoop.cfs.CassandraFileSystem</value>
    </property>
  3. Save the file and restart Cassandra.

    DSE creates the new CassandraFS.

  4. To access the new CassandraFS, construct a URL using the following format:
    cfs-<filesystemname>:<path>

    For example, assuming the new file system name is NewCassandraFS use the dse commands to put data on the new CassandraFS.

    dse hadoop fs -put /tmp/giant_log.gz cfs-NewCassandraFS://cassandrahost/tmp
    
    dse hadoop fs distcp hdfs:/// cfs-NewCassandraFS:///
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