Seems like nowadays, everyone's saying SQL database do not scale, while apparently, most crowded sites are using mySQL (or other SQL based) and scaling it quite well, producing speedy websites that serve billions of pages everyday.
Serving billions of pages a day is not a particularly hard scaling problem if most of those page views only require reads. It's writes, and particularly transactional updates, that are hard.
yes, but even harder are the entire set of problems where the data is extremely interrelated, like Facebook's is -- making it very difficult to shard effectively.
