well, that's how it works i guess... probably can get a lot more out of your disks by spending instead on memory, if the kernel is tweaked to avoid flushing the cache as much, and you have good power backup
Discussion
Right now I switched over to truenas core as my SAN device with 128gb of memory cache, so hopefully I see good results. I have over 1TB of memory in the rack so I got plenty to spare. Truenas has consumed almost all of it.