fantastic question you have there pal, lots of people may be interested in this.
i will provide my input for you.
now a lot of people get stressed over how much cache memory a hard drive has and its effect on the performance of the drive. now the thing is, the cache can be used for reads and writes, but the true defining factor of a drives speed will be the rotation speed and the platter density.
when doing sustained reads, e.g pulling a 4gig file off the drive, cache probably wont be used at all since it will be the drives mechanical limits at effect. pulling dats off the drive at 90mb/s or so and same with a sustained write the mechanics of the drive are the limiting factor.
now only way i can see a cache having impact on performance is if you use native command queuing on a sata drive for example. lets say the drive is in a nas box and 3 people are pulling different files from the drive, with more cache memory there can be a larger queue depth in the buffer for how the reads are to be sequenced for optimal performance or minimal drive wear. same with writes, can have alonger queue etc.
thing is storagereview.com and few other sites have shown some tests with drives, some identical models but different cache sizes made far bit of difference back in the day. going from 2mb cache to 8mb cache was shown to have like 4% increase in performance, going from 8 to 16mb cache had even less of an impact. and from 16 to 32mb cache will probably be even less of a difference then going from 8 to 16mb was.
one negative i can think of is if you have a large cache size and your writing data to the drive and you have a power cut. you could potentially loose the data that was in the buffer at the time. how big is a average word document or excel spreadsheet in size?
i know i havent answered your question as such but if i were to sum it up id say 16 to 32mb cache increase would probably not be noticed at all in everyday machines. possibly in certain IO benchmarks the difference may be seen.
main reason cache sizes increase is probably due to marketing. e.g WD pull out a 8mb cache drive all other makers will follow or thier product looks lame to the average high street punter.
suffice to say cache is dirt cheep and i wont be surprised to see a 64mb cache drive out at some point in time. and all manufacturers will jump on that or they look outdated to the average punter again.
