zlacker

The terms are used loosely and it doesn't make a lot of sense to argue about the definitions.

I think it's true to say that vertical scaling normally is done by increasing the RAM and CPU of a single machine with a single address space and switch/bus. While horizontal scaling is normally adding more machines (additional addresses spaces and switch/bus). Historically this is because RAM to CPU performance (throughput and latency) in a single address space and bus greatly exceeds the performance of any NIC connecting machines with distinct address spaces and busses. And it mostly ignores effects like the performance costs of swapping/paging when you don't have enough RAM.

I haven't really seen many systems where horizontal scaling is truly linear, unless the problem is embarassingly parallel, like serving static content.

>>dekhn+(OP)
Note that I was referring to scaling of cost, not of performance. If your application parallelizes ideally, then in the worst case your cost will scale linearly, because you just add more machines and increase your power consumption by new_machine_count/previous_machine_count. It's possible adding more processors in the same address space increases the cost by an amount below new_core_count/previous_core_count, in which case the cost scales better than linearly.