What do you think about this point, and how does Apple address it in the Fusion drive?"A big difference between cache and automated tiering is that the data in cache is always a second copy of the data that is on the hard drive. Automated tiering is an actual move of data from the hard drive. Failure of the cache rarely produces a data loss, just a performance loss since everything would need to be served from mechanical drives until the cache can be replaced.Since the SSD tier holds potentially the only copy of data in an automated tiering system, the failure of the SSD tier can't be tolerated so these systems have to set the SSD tier in a redundant configuration by using a RAID-like data protection scheme. The overhead of that protection, RAID parity bit calculation for example, may impact performance and of course any RAID algorithm requires extra disk capacity. Having to purchase extra SSD to support a RAID-like function makes an already premium priced technology even more expensive."
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