1) If you have all the data to load into place, you can try sstableloader (only for cassandra 0.8.x and beyond) to bulk load the data. Read more: cassandra bootloader
2) Cassandra introduced BulkOutputFormat bulk data loading into cassandra using the hadoop job in the latest version of cassandra-1.1.x and beyond. See Downloading to Kassandra using Hadoop for more details.