> **Key Takeaways**
>
> * Docker container console logs are stored as files on the host's disk, **not in system memory**.
> * The `docker logs` command reads and displays these stored files.
> * To prevent log files from growing indefinitely, it's crucial to **configure log rotation options (`max-size`, `max-file`)**.

## 1. Where and How Are Logs Stored? {#sec-37c2a29cff35}


Docker captures both standard output (stdout) and standard error (stderr) streams from containers and processes them via a **Logging Driver**. If no specific configuration is made, Docker defaults to using the `json-file` logging driver.

*   **Storage Location**: Container logs are stored as JSON-formatted files in a specific path on the host system. The typical path is:
    *   `/var/lib/docker/containers/[컨테이너_ID]/[컨테이너_ID]-json.log`
*   **`docker logs` Command**: This command doesn't display content from memory; rather, it reads the aforementioned JSON log files and presents them in the terminal.

![Docker container log file example](/media/whitedec/blog_img/ff3498a786c645ecb031d714a053d4a4.webp)

## 2. What if Logs Keep Accumulating and Cause Disk Space Issues? {#sec-4e6257ec2aba}


Yes, that's correct. If no action is taken, log files can grow indefinitely and consume all of the host's disk space. To prevent this, Docker provides **Log Rotation** options to limit the size and number of log files.

For example, you can add the following options when running a `docker run` command:

```
docker run -d \
  --log-driver json-file \
  --log-opt max-size=10m \
  --log-opt max-file=3 \
  nginx
```

*   `--log-opt max-size=10m`: Limits the maximum size of a single log file to **10MB**.
*   `--log-opt max-file=3`: Limits the maximum number of log files to **3**. If the number exceeds 3, the oldest file is deleted.

With this setting, the logs for that container will use a maximum of 30MB (10MB * 3 files) of disk space.

***

## 3. Configuring in `docker-compose.yml` {#sec-890442ea3ac0}


With Docker Compose, you can easily configure logging options for each service directly within the `docker-compose.yml` file.

Think of it as the `--log-opt` flags from the `docker run` command, but in YAML format.

Here's an example of applying log rotation to a service named `my-app`:

```
services:
  my-app:
    image: your-app-image
    # ... other configurations ...
    deploy:
      resources:
        limits:
          memory: 512M

    # 👇 This section configures logging.
    logging:
      driver: "json-file"  # The logging driver to use (can be omitted as it's the default)
      options:
        max-size: "10m"    # Maximum size per file: 10MB
        max-file: "3"      # Maximum number of files: 3
```

*   **`logging`**: This key initiates the logging configuration for the service.
*   **`driver`**: Specifies the logging driver to use. `json-file` is the default.
*   **`options`**: Configures options to be passed to the driver.
    *   **`max-size`**: Identical to `docker run`'s `--log-opt max-size`.
    *   **`max-file`**: Identical to `docker run`'s `--log-opt max-file`.

Now, when you start this service with `docker-compose up`, the `my-app` container's logs will automatically rotate, using a maximum of 30MB (10MB * 3 files) of disk space.

## 4. Setting Defaults for All Containers {#sec-1a0729e53c34}


If you find it cumbersome to apply options to each container individually and wish to enforce a default policy across the entire system, modifying the **Docker Daemon configuration** file is the best approach.

1.  Open or create the `/etc/docker/daemon.json` file. (If the file doesn't exist, create a new one.)
2.  Add the following content:

```json
{
    "log-driver": "json-file",
    "log-opts": {
        "max-size": "10m",
        "max-file": "3"
    }
}
```

3.  Restart the Docker daemon.

```bash
sudo systemctl restart docker
```

With this configuration, **all containers** created in the future will automatically adhere to this policy unless specific logging settings are overridden. Of course, if you specify different logging options for a particular container in a `docker-compose.yml` file or via the `docker run` command, **individual settings will take precedence** over the daemon's default configuration.

*   **Per-service Configuration**: Setting logging options within the `logging` section of a `docker-compose.yml` file is generally the most common and convenient method.
*   **Global Default Configuration**: To apply a uniform policy to all containers on the server, modify the `/etc/docker/daemon.json` file.