Doesn't a hard drive use its cache with high utilization? I see up to 20MB/s difference on file writes when I disable windows disk write cache. Does anybody know why cache size matters this much?
On a healthy Linux system, I see about half the RAM used by disk cache and buffers. That's several orders of magnitude higher than the amount of cache in hard disks. Furthermore, access to RAM is faster than access to disk cache. Furthermore, the OS has information on what each disk sector is used for and can fine-tune its caching strategy (e.g. weigh certain files or directories as more likely to be needed again and keep them in cache longer). The in-disk disk cache isn't nearly as important for performance as the OS disk cache; it's mostly useful when the OS needs to write a large burst of data (the OS writes to the disk cache quickly, and the disk takes its time flushing the data to the actual permanent storage).