In the previous step, you created a Dockerfile and executed a command. Now, you’ll learn how to persist data outside of containers.
This walkthrough teaches you how to persist data outside of container. Runpod has the same concept used for attaching a Network Volume to your Pod.Consult the documentation on attaching a Network Volume to your Pod.

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Why persist data outside a container?

The key goal is to have data persist across multiple container runs and removals. By default, containers are ephemeral - everything inside them disappears when they exit. So running something like: 
docker run busybox date > file.txt
Would only write the date to file.txt temporarily inside that container. As soon as the container shuts down, that file and data is destroyed. This isn’t great when you’re training data and want your information to persist past your LLM training. Because of this, we need to persist data outside the container. Let’s take a look at a workflow you can use to persist data outside a container.

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Create a named volume

First, we’ll create a named volume to represent the external storage: 
docker volume create date-volume

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Update Dockerfile

Next, we’ll modify our Dockerfile to write the date output to a file rather than printing directly to stdout: 
FROM busybox
WORKDIR /data
RUN touch current_date.txt
COPY entrypoint.sh /
RUN chmod +x /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]
This sets the working directory to /data, touches a file called current_date.txt, and copies our script.

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Update entrypoint script

The entrypoint.sh script is updated: 
#!/bin/sh
date > /data/current_date.txt
This will write the date to the /data/current_date.txt file instead of printing it.

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Mount the volume

Now when the container runs, this will write the date to the /data/current_date.txt file instead of printing it. Finally, we can mount the named volume to this data directory: 
docker run -v date-volume:/data my-image
This runs a container from my-image and mounts the date-volume Docker volume to the /data directory in the container. Anything written to /data inside the container will now be written to the date-volume on the host instead of the container’s ephemeral file system. This allows the data to persist. Once the container exits, the date output file is safely stored on the host volume. After the container exits, we can exec into another container sharing the volume to see the persisted data file: 
docker run --rm -v date-volume:/data busybox cat /data/current_date.txt
This runs a new busybox container and also mounts the date-volume.
  • Using the same -v date-volume:/data mount point maps the external volume dir to /data again.
  • This allows the new container to access the persistent date file that the first container wrote.
  • The cat /data/current_date.txt command prints out the file with the date output from the first container.
  • The --rmflag removes the container after running so we don’t accumulate stopped containers.
Remember, this is a general tutorial on Docker. These concepts will help give you a better understanding of working with Runpod.