scale out VS. scale up.What is Database Scale-Out?

scale out (by increasing the number of servers)
scaling up (by adding more memory and CPUs to the servers)


What is Database Scale-Out?
Scale-Out is a modern computing architecture that enables
organizations to improve application performance and scalability on an
incremental, as-needed basis by adding multiple replicated database
servers on low-cost commodity hardware. This is in contrast to a
Scale-Up approach, which requires organizations to make a large
up-front investment in more expensive and complex server hardware and
database licenses in order to add capacity.
In the online world,
many of the largest and fastest-growing companies use MySQL to
cost-effectively Scale-Out their successful businesses, saving millions
of dollars over high-cost proprietary technology — including Google,
Yahoo, craigslist, Ticketmaster, Wikipedia, YouTube, and
Evite/Citysearch.