Docker buildkit cache location/size and ID

Question

There's Docker buildkit: https://github.com/moby/buildkit/blob/master/frontend/dockerfile/docs/experimental.md which has an extra option to RUN (amongst others): the --mount=type=cache.

What I couldn't figure out from the documentation is:

1. where is that cache stored?
2. how can one delete/inspect that?
3. how does buildkit decide what cache goes where? For example if I have the same Dockerfile in two locations, will the caches be the same? What's the key for the cache?
4. the ID option is still ambiguous. If I specify the same ID in different dockerfiles, will they refer to the same cache?

Answer 1

Yes, it is somewhat vague in docker 20.10.5. Could use a pull request or two to update documentation.

1. The cache uses the same storage driver as used for image layers. Metadata is stored in databases at /var/lib/docker/buildkit. When using overlay2 driver the layer itself is in /var/lib/docker/overlay2/<ID>/diff/. For <ID>, see below. /var/lib/docker can vary depending on data-root in your dockerd configuration.
2. docker buildx du --verbose lists build cache. You can also inspect it from docker system df -v --format '{{ .BuildCache | json }}'. The cache type exec.cachemount is the RUN --mount type=cache. You can find the layer using the ID, which is not the same as used in --mount id. The mount type is implemented by buildkit, so the docker run --mount does not recognize it. To get rid of it either docker buildx prune or docker build --no-cache.
3. The cache key is the value from id=. id defaults to value of target. You need to specify id when you need different cache at the same target.
4. Yes. They are the same cache regardless of the target or Dockerfile. Different builders have their own caches, which keeps for example caches for different architectures separate.

Answer 2

For #3 it seems when you don't specify an id to the --mount option, the same cache will be used, regardless of any other parameters, like the mountpoint or the Dockerfile contents/path.