How is data written?

Logging writes and memtable storage

When a write occurs, the database stores the data in a memory structure called memtable. To provide configurable durability, the database also appends writes to the commit log on disk. When commitlog_syncis set to sync, the commit log receives every write made to a node. These durable writes survive permanently even if power fails on a node. The memtable is a write-back cache of data partitions that the database looks up by key. The memtable stores write operations in sorted order until reaching a configurable limit, and then is flushed.

Flushing data from the memtable

To flush data from the memtable, the database writes data to disk in the memtable-sorted order. A partition index is also created on the disk that maps the tokens to a location on disk.

When the memtable content exceeds the configurable threshold, or the commitlog space exceeds the commitlog_total_space_in_mb, the memtable is put in a queue that is flushed to disk. If the data to be flushed exceeds the memtable_cleanup_threshold (which is automatically calculated), the database blocks writes until the next flush succeeds.

You can manually flush a table using nodetool flush or nodetool drain (flushes memtables without listening for connections to other nodes). To reduce the commit log replay time, DataStax recommends flushing the memtable before you restart the nodes. If a node stops working, replaying the commit log restores the writes to the memtable that were there before the node stopped.

Purging commit log segments

The database uses the commit log to rebuild memtables. The commit log is divided into segments. Writes are recorded in order and new segments are created when the current segment reaches the commitlog_segment_size_in_mb. The database purges commit log segments only after all the data in a segment has been flushed to disk from the memtable. If the commit log directory reaches the maximum size (commitlog_total_space_in_mb), the oldest segments are purged and the corresponding tables are flushed to disk.

All the commit log segments contain writes for both table A and table B, as well as system tables. Table A's memtable fills up rapidly and gets flushed frequently; while table B's memtable fills up slowly and is rarely flushed. When the commit log reaches the maximum size, it forces Table B's memtable to flush and then purges the segments.

Table B is flushed into large chunks instead of hundreds of tiny SSTables. If the commit log space and memtable space are equal, Table B's memtable would flush every time Table A is flushed, despite being much smaller. To summarize, if there is more than one table, it makes sense to have a larger space for commit log segments.

Storing data on disk in SSTables

Memtables and SSTables are maintained per table. The commit log is shared among tables. SSTables are immutable, not written to again after the memtable is flushed. Consequently, a partition is typically stored across multiple SSTable files. A number of other SSTable structures exist to assist read operations.