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In simplified terms, cache on HDD's is used to overcome limitations of the mechanics native to HDD technology. The HDD is the slowest main device in the process that the system uses for almost everything like the CPU, RAM, and Video Card. Without the cache the system would have to wait more often than not for the drive to spin to a location to either read or write data before it could handle the next request. That spinning to the location is called latency or random access time. Cache allows a list of commands to be stored in a place the drive can pull from. This allows the cpu, ram, and video card to continue working rather than have to stop and wait on the HDD. Now because the read and write locations are often all over the place on the drive at anyone time, the drive will fluctuate in performance which can also be modified by the cache. This allows the Drive controllers to optimize performance based on load since it can predict what the drive can handle based on the current list of read write commands in the cache.
What this means to you. Often more cache can make up for drives with less efficient controllers or less cache can reduce cost when coupled with efficient controllers on the drives. The Spin Points seem to show the later based on benchmarks shown often online. It's a balance more than anything and each drive manufacturer has their own philosophies on which is the better solution. WD seems to trend toward the greater cache to compensate for less efficient controllers. Overall the easiest way to look at it is drives are like miniature computers and cache is comparable to system ram. If you have intensive operations such as raid, then the cache will help you more than the controller more often than not. If you have standard operations that process quickly then the more efficient drive controller will benefit you more.
Thanks, I appreciate the information. I will compare the differences in cost for various disk cache sizes, and then make a decision based on my needs in terms of the types of operations performed.