---
title: "Kafka log segments on disk: leader vs follower"
description: "Nine records roll three segment files named by base offset. A stalled follower holds less data than the Kafka leader, and retention deletes whole files."
url: https://monedula.dev/kafka-simulator/scenarios/partition-on-disk/
---

# Kafka log segments on disk: leader vs follower

Scenario 13.1.3 · A partition on disk, leader vs follower. Terminal ops · Intermediate · about 3 min · Apache Kafka 4.3.

A partition is a directory of segment files named by base offset: watch them roll, see a stalled follower hold fewer bytes than its leader, and watch retention delete a whole file.

Run it: https://monedula.dev/kafka-simulator/playground/?scenario=13.1.3

## What you start with

This scenario is played from the terminal (Ctrl and the backtick key). A partition is not an abstraction on disk: it is a directory of segment files, one per broker that hosts a replica, and the file names are offsets. Nine records roll three segments, a stalled follower falls behind mid-stream, and then retention runs.

## Step by step

1. **An empty log directory** (t+0.000s): Open the terminal with Ctrl and the backtick key (on a phone, the "..." menu). You start on broker-1, inside its log root. `tree` draws what a broker actually keeps on disk: one directory per partition replica it hosts, and inside it an active segment file that already exists while holding nothing. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=13.1.3&t=0)
2. **Six records, and the log has rolled** (t+2.700s): Six records are in. `ls -lh orders-0` shows two segment files where there was one: a file is named after the FIRST offset it can hold, zero padded to twenty digits, so the names are a map of the log. Open the closed one with `kafka-dump-log --files orders-0/00000000000000000000.log --print-data-log` and read the batch header and the records inside it. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=13.1.3&t=1)
3. **Nine records, three segments** (t+4.200s): Three more records rolled a third file. `ls -lh orders-0` lists them with the sizes they carry, `kafka-dump-log --files orders-0/00000000000000000004.log --print-data-log` reads the middle one, and `du -sh orders-0` totals what this partition costs broker-1. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=13.1.3&t=2)
4. **The same partition, on a stalled follower** (t+4.600s): broker-3 stopped fetching in the middle of the stream. `ssh broker-3` stands you on the follower, then run `ls -lh orders-0` there: same directory, same naming, less data. Its newest segment is short and the leader's third file does not exist on this machine at all, and `du -sh orders-0` prices the difference. A replica this far behind is also out of the ISR, which `kafka-topics --describe --topic orders` confirms; at RF 3 with min.insync.replicas 2 the writes kept succeeding anyway. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=13.1.3&t=3)
5. **Retention deleted a file, not records** (t+8.200s): A retention run, scripted here rather than triggered by the 60 second retention.ms window, advanced the log start to offset 4, which is a segment boundary. `exit` back to broker-1 and run `ls orders-0`: the first segment did not shrink, it is gone, and so are its index files. `kafka-get-offsets --topic orders --time earliest` reports the new floor the deletion left behind. [Open at this step](https://monedula.dev/kafka-simulator/playground/?scenario=13.1.3&t=4)

## The takeaway

A segment file is named after the FIRST offset it can hold, so the file names are a map of the log. The directory belongs to a REPLICA, not to the partition: a follower that stops fetching keeps a shorter active segment than its leader, and the only way to see that is to stand on the follower. And retention works in whole files - advancing the log start to a segment boundary makes a file disappear from ls rather than shrink, which is why a retention window can only ever be approximate.

## Configs in this scenario

- [`min.insync.replicas`](https://kafka.apache.org/43/configuration/topic-configs/#topicconfigs_min.insync.replicas): The fewest in-sync replicas, leader included, that must have a write for an acks=all produce to succeed (default 1); with fewer in the ISR the leader rejects the write with NOT_ENOUGH_REPLICAS. [How min.insync.replicas works](https://monedula.dev/kafka-simulator/configs/min-insync-replicas/)
- [`retention.ms`](https://kafka.apache.org/43/configuration/topic-configs/#topicconfigs_retention.ms): How long a topic with cleanup.policy=delete keeps a log segment before it may be deleted (default 604800000, 7 days); -1 removes the time limit. [How retention.ms works](https://monedula.dev/kafka-simulator/configs/retention-ms/)

## Further reading

- [Apache Kafka documentation](https://kafka.apache.org/43/configuration/topic-configs/#topicconfigs_retention.ms)