Set the key/value in our lower level storage unconditionally. This might be important for permenant storage hierarchies or higher read performance (at the cost of write performance and space).
Only update lower level storage if the key currently resides in them. The next get on the key will be forced to retrieve from the higher level, so this maintains consistency while not requiring extra storage (at the cost of performance).
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When an update arrives from a higher level cache, how should we update lower level caches?