Cassandra backs up data by taking a snapshot of all on-disk data files (SSTable files) stored in the data directory.
You can take a snapshot of all keyspaces, a single keyspace, or a single table while the system is online.
Using a parallel ssh tool (such as pssh), you can snapshot an entire cluster. This provides an eventually consistent backup. Although no one node is guaranteed to be consistent with its replica nodes at the time a snapshot is taken, a restored snapshot resumes consistency using Cassandra's built-in consistency mechanisms.
After a system-wide snapshot is performed, you can enable incremental backups on each node to backup data that has changed since the last snapshot: each time an SSTable is flushed, a hard link is copied into a /backups subdirectory of the data directory (provided JNA is enabled).