Managing cache
Earthly cache works similarly to Dockerfile layer-based caching. In fact, the same technology is used underneath.

Cache layers

Many Earthfile commands create cache layers. A cache layer may be reused in a future build, if the conditions under which it is created are the same.
Examples of commands which create layers are COPY and RUN.
One of the main things influencing the conditions are "sources". Sources are created through commands like COPY and GIT CLONE. RUN, however, is not a source, even if the command itself involves downloading content from an external location. This means that a RUN command, on its own, would always be cached if it has been run under the same circumstances previously (except for the RUN --push variant).
For a primer into Dockerfile caching see this article. The same principles apply to Earthfiles.

Cache location

Earthly cache is persisted in a docker volume called earthly-cache on your system. When Earthly starts for the first time, it brings up a BuildKit daemon in a Docker container, which initializes the earthly-cache volume. The volume is managed by Earthly's BuildKit daemon and there is a regular garbage-collection for old cache.

Checking current cache size

You can check the current size of the cache by running:
sudo du -h /var/lib/docker/volumes/earthly-cache | tail -n 1

Specifying cache size limit

The default cache size is adaptable depending on available space on your system. If you would like to limit the cache size more aggressively, you can specify a different limit by modifying the cache_size_mb and/or cache_size_pct settings in the configuration. For example:
global:
cache_size_mb: 10000
cache_size_pct: 70

Resetting cache

The cache can be safely deleted manually, if the daemon is not running
docker stop earthly-buildkitd
docker rm earthly-buildkitd
docker volume rm earthly-cache
However, it is easier to simply use the command
earthly prune --reset
which restarts the daemon and resets the contents of the cache volume.

See also

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Cache layers
Cache location
Specifying cache size limit
Resetting cache
See also