By default, the build system will use a shared state cache directory called sstate-cache on your build directory to store the cached data. This can be changed with the SSTATE_DIR configuration variable in your conf/local.conf file. The cached data is stored in directories named with the first two characters of the hash. Inside, the filenames contain the whole task checksum, so the cache validity can be ascertained just by looking at the filename. The build process set scene tasks will evaluate the cached data and use it to accelerate the build if valid.

When you want to start a build from a clean state, you need to remove both the sstate-cache directory and the tmp directory.

You can also instruct BitBake to ignore the shared state cache by using the --no-setscene argument when running it.

It's a good practice to keep backups of clean shared state caches (for example, from a build server), which can be used in case of shared state cache corruption.