For some reason you'll rarely see this mentioned, but the primary benefit of the cache is to reduce the number of time-intensive disk head movements. Picture this scenario, one file is being read, and another file is being written at the same time. Without a cache, the head would be jumping all over the place trying to service these independent file streams. However, with a cache, the file being written can be temporarily stored in the cache, while the other file being read continues to be pulled from disk. If the file being written fits entirely in the cache, the hard drive signals to the OS that the write has completed, allowing the program that was writing the file to continue, despite that the disk cache hasn't yet completed writing the file to the physical disk. Meanwhile, the reading of the first file never stopped. In this way, significantly higher apparent bandwidth was accomplished, to the extent of the cache size. This may not be significant when dealing with 2 MB caches, but newer large capacity drives have 256 MB caches, which is enormous, and even under high read/write loads, will give highly optimized read throughput (very close to the physical disk's rating) while simultaneously supporting full SATA write bandwidth, until you run out of cache. Usually, you'd balance the number of drives in your RAID to match your throughput requirements so you don't typically exceed your cache capacity.
I agree with this comment. I have seen a significant difference when using a WDC 5400 rpm drive vs. a 7200 rpm drive but no noticeable difference with two like speed drives using different cache size with drives running at the same spindle speed. – jtreser – 2010-02-12T10:57:22.597
Agreed that at around $3(?) why not. Likewise agreed that there is very little difference in how fast it will go. More important is the spin speed. – ssvarc – 2010-02-12T11:07:40.323
@JeffAtwood, What do you mean by "firmware optimizations"? Are there even real harddisk algorithm optimizations in this time and age? Don't they all use the same standard algorithm these days? – Pacerier – 2016-12-18T12:36:06.230