You may remember how in a [previous post]({{< ref "docker-images-layers-and-cache/" >}}) we used a small example to discuss layers and final image size. Well, here's our example again, slightly modified.
Given that the base image weighs ~75MB, we could expect the final image to weigh ~175MB (~75 from the base image + ~100 from the big file we generated).
It turns out that since `chown`ing the file modifies it, the `example` file will count twice: once in the `fallocate` layer, and once in the `chown` layer, resulting in an image size of ~275MB.
## 📉 Workaround
Since creating "large" amounts of data in a Docker image can be quite common (think about dependencies, static files, etc), I guess that workaround strategies are required. Fortunately, our backs are covered.
Let's take a slightly more complex example to illustrate some real life situations you might encounter:
```Dockerfile
FROM ubuntu AS build
WORKDIR /build
RUN fallocate -l 100M binary
FROM ubuntu
WORKDIR /app
RUN fallocate -l 100M example
COPY --from=build /build/binary /app/binary
RUN chown -R 33:33 /app
```
This results in an image weighing 492MB. Let's bring it down to 283MB! (2x~100MB + ~75MB)
There you go! By being smart about when to run the permission changes, we just saved ourselves 200MB of disk space and network bandwidth. That's about 60% for this specific image!
In the specific case I was investigating at [ITSF](https://itsf.io), the image went from ~1.6GB to ~0.95GB just from this `chown` trick. We were copying a bunch of files in a directory and at the end we chowned the whole directory recursively. That directory weighed about 650MB, which counted twice in the final image size.
I discovered that the `chown` was taking that much space using the underrated `docker history` command. I already briefly [introduced]({{< ref "docker-images-layers-and-cache/#cache-invalidation" >}}) it previously but now felt like a good time to remind you of its existence 🙂
<missing> 3 months ago /bin/sh -c set -xe && echo '#!/bin/sh' > /… 811B
<missing> 3 months ago /bin/sh -c #(nop) ADD file:4f15c4475fbafb3fe… 72.9MB
```
All the `<missing>` rows plus the first row with a real ID above (`f643c72bc252`) are the layers of the base image. All the layers above are the ones that compose our image. We can clearly see that the `chown` layer weighs 210MB by itself.
