Taking a snapshot
Snapshots are taken per node using the nodetool snapshot command. To take a global snapshot, run the nodetool snapshot command using a parallel ssh utility, such as pssh.
A snapshot first flushes all in-memory writes to disk, then makes a hard link of the SSTable files for each keyspace. By default the snapshot files are stored in the /var/lib/cassandra/data/<keyspace_name>/<table_name>/snapshots directory.
You must have enough free disk space on the node to accommodate making snapshots of your data files. A single snapshot requires little disk space. However, snapshots can cause your disk usage to grow more quickly over time because a snapshot prevents old obsolete data files from being deleted. After the snapshot is complete, you can move the backup files to another location if needed, or you can leave them in place.
$ nodetool -h localhost -p 7199 snapshot demdb