It's not even the percentage of reads vs writes; it's about the total time spent writing. Suppose that while a single user was actively using the website, the

It's not even the percentage of reads vs writes; it's about the total time spent writing. Suppose that while a single user was actively using the website, the time spent writing was 70%. That's a totally mad write load; but even then, if you had two concurrent users, things would probably still be quite useable -- you'd have to go up to 4 or 5 users before things really started to slow down noticeably. Suppose, on the other hand, that a single user generated around a 1% write utilization when they were actively using the website (which still seems pretty high to me). You could probably go up to 120 concurrent users quite easily. And given that not all of your users are going to be online at exactly the same time, you could probably handle 500 or 1000 total users.