There are many design decisions that contributed to Kafka’s performance. In this post, we’ll focus on two. We think these two carried the most weight.

Sequential I/O

The first one is Kafka’s reliance on Sequential I/O.

Zero Copy

The second design choice that gives Kafka its performance advantage is its focus on efficiency: zero copy principle.

The diagram above illustrates how the data is transmitted between producer and consumer, and what zero-copy means.

• Step 1.1 - 1.3: Producer writes data to the disk
• Step 2: Consumer reads data without zero-copy
  • 2.1: The data is loaded from disk to OS cache
  • 2.2 The data is copied from OS cache to Kafka application
  • 2.3 Kafka application copies the data into the socket buffer
  • 2.4 The data is copied from socket buffer to network card
  • 2.5 The network card sends data out to the consumer
• Step 3: Consumer reads data with zero-copy
  • 3.1: The data is loaded from disk to OS cache
  • 3.2 OS cache directly copies the data to the network card via sendfile() command
  • 3.3 The network card sends data out to the consumer

Zero copy is a shortcut to save multiple data copies between the application context and kernel context.