The default behavior of Poky is to build everything from scratch unless BitBake determines that a recipe does not need to be rebuilt. The main advantage of building everything from scratch is that the final result is fresh and there is no risk of previous data causing problems. However, rebuilding everything requires computational time and resources.

The strategy to determine whether a recipe must be rebuilt is complex. Basically, BitBake tries to track as much information as possible about every task, variable, and code used in the build process. BitBake then generates a checksum for all the involved information for every task.

Poky uses all this information provided by BitBake to store snapshots of those tasks as a set of packaged data generated in a cache, which is called the shared state cache (sstate-cache). This cache wraps the contents of each task output in packages stored in the SSTATE_DIR directory. Whenever BitBake prepares to run a task, it first...