---
title: 'Why Is Kafka Fast? The Secret Behind Its High Throughput'
source: 'https://youtube.com/watch?v=wvLdBJEl-wc'
video_id: 'wvLdBJEl-wc'
date: 2026-09-03
duration_sec: 125
channel: 'ByteByteGo'
---

# Why Is Kafka Fast? The Secret Behind Its High Throughput

> Source: [Why Is Kafka Fast? The Secret Behind Its High Throughput](https://youtube.com/watch?v=wvLdBJEl-wc)

## Summary

This video explains why Apache Kafka is considered fast, focusing on its design for high throughput rather than low latency. It highlights two key design decisions: reliance on sequential I/O and the use of an append-only log, which enable efficient data movement and cost-effective long-term message retention.

### Key Points

- **Defining 'Fast' in Kafka** [00:00] — The term 'fast' is ambiguous; Kafka is optimized for high throughput, moving large numbers of records quickly, akin to a large pipe for liquid.
- **Two Key Design Decisions** [00:29] — Kafka's performance stems from many design choices, but two are highlighted: sequential I/O and the append-only log.
- **Sequential I/O vs. Random Access** [00:44] — Sequential access is much faster than random access on hard disks because the arm doesn't need to jump around; random access is slow due to physical movement.
- **Append-Only Log** [01:08] — Kafka uses an append-only log as its primary data structure, adding new data to the end of the file, which is a sequential access pattern.
- **Performance Numbers** [01:24] — On modern hardware, sequential writes can reach hundreds of MB/s, while random writes are in hundreds of KB/s—several orders of magnitude difference.
- **Cost Advantage of Hard Disks** [01:39] — Hard disks cost one-third of SSDs but offer three times the capacity, allowing Kafka to retain messages cost-effectively for long periods without performance penalty.

### Conclusion

Kafka's speed is rooted in its throughput-oriented design, leveraging sequential I/O and append-only logs to achieve high performance and cost-effective data retention, a feature uncommon in earlier messaging systems.

## Transcript

Why is Kafka fast? What is the secret? We'll talk about it in this video. Let's dive right in. We'll first start by acknowledging that the term fast is ambiguous. What does it even mean that Kafka is fast? Are we talking latency? Are we talking throughput? Is fast compared to what?
Kafka is optimized for high throughput. It is designed to move a large number of records in a short amount of time. Think of it as a very large pipe for moving liquid. The bigger the diameter of the pipe, the larger the volume of liquid that can move through it. So when someone
says Kafka is fast. They usually refer to Kafka's ability to move a lot of data efficiently. What are some of the design decisions that help Kafka move a lot of data quickly? There are many design decisions that contributed to Kafka's performance. In this video, we'll focus on two. We think these
two carry the most weight. The first one is Kafka's reliance on sequential I.O. Now what is sequential I.O.? Let's dig into it a little bit. There's a common misconception that this access is slow compared to memory access But this largely depends on the data access pattern There are two common disk access patterns random and sequential For hard drives it takes time to physically move the arm to different locations on the magnetic disks
This is what makes random access slow. For sequential access though, since the arm doesn't need to jump around, it is much faster to read and write blocks of data one after the other. Kafka takes advantage of this by using an append-only log as its primary data structure. An append-only
log adds new data to the end of the file. This access pattern is sequential. Now let's bring this idea home with some numbers. On modern hardware with an array of these hard disks, sequential writes can reach hundreds of megabytes per second, while random writes are measured in
hundreds of kilobytes per second. Sequential access is several orders of magnitude faster. Using hard disks has its cost advantage too. Compared to SSD, hard disks come as one-third of the price but with about three times the capacity giving Kafka a large pool of cheap disk
space without the performance penalty means that Kafka can cost effectively retain messages for a long period of time. This is a feature that was uncommon to messaging system before Kafka.