Two proposals for a layered store with two lower stores:
- one lower (called lower_new) is used until it is full, at which point the other lower (called lower_old) is cleared and it becomes lower_new:
- one lower contains everything (called lower_all), while a second lower (called lower_latest) contains only the latest additions. lower_latest is cleared and refilled at each freeze.
In both cases, each lower is self contained, but not the upper.