From Cassandra's presentation slides (slide 2) link 1, alternate link:
scaling writes to a relational database is virtually impossible
I cannot understand this statement. Because when I shard my database, I am scaling writes isn't it? And they seem to claim against that.. does anyone know why isn't sharding a database scaling writes?
It is not. The slide is wrong (or at least the statement ought to be more carefully qualified when making such an apparently bold claim).
What it means is that some SQL-based products are not a good fit for some of those high scalability scenarios. To assume that any or all "relational databases" will have the same problems would be a gross over-generalisation. Unfortunately it's just the kind of over-generalisation that the No-SQL marketing crowd have become notorious for.