Each Kafka partition is stored as a sequence of segment files on disk. Kafka only appends — it never modifies or deletes individual messages. This makes writes extremely fast (sequential I/O) and enables OS page cache exploitation. Sequential disk writes on modern SSDs rival in-memory performance at a fraction of the cost.
Key configuration
log.dir=/var/kafka/logs — data directory
log.segment.bytes=1073741824 — 1GB per segment (default)
log.roll.ms — time-based segment rollover