How is data written

Data written to a Cassandra node is first recorded in an on-disk commit log and then written to a memory-based structure called a memtable. When a memtable's size exceeds a configurable threshold, the data is written to an immutable file on disk called an SSTable. Buffering writes in memory in this way allows writes always to be a fully sequential operation, with many megabytes of disk I/O happening at the same time, rather than one at a time over a long period. This architecture gives Cassandra its legendary write performance.

Cassandra processes data at several stages on the write path, starting with the immediate logging of a write and ending in with a write of data to disk:

• Logging data in the commit log
• Writing data to the memtable
• Flushing data from the memtable
• Storing data on disk in SSTables