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If I understand it correctly, the main disadvantage of snooping based cache coherence is that, whenever a processor does a cache operation which may affect coherence, the message has to be broadcast to all other processors via bus. This doesn't scale as the number of processors goes up and creates lots of messages between processors on the bus, since bus is also a shared resource. On the other hand, directory based cache coherence stores the coherence information in a directory, which requires more storage on the processor but largely reduce the messages overhead introduced by the coherence protocol.