Scalability of Ruby on Rails

I was looking through the slides from jcon today.  I really wish I could have gone; it looked like it was awesome.

I noticed that Yehuda Katz was a speaker.  He's also a member of the Ruby on Rails core team, and I have been hearing rumors that Ruby on Rails doesn't scale well.

That rumor is a thorn in my side because I am interested in learning Ruby on Rails, but I want to be able to use it on a potentially enterprise level...

I sent Yehuda Katz an email, complimenting his slides and asking about the scalability of Rails and telling him that I had heard it didn't.

He replied with the following, and I thought it would be a good idea to share, just so anyone else who is interested could add their input.

"People who say this don't really understand what scaling is. Scaling is not about the performance of the VM, it's about fundamental architecture. Rails uses the REST architecture, the same architecture used by the Internet itself. It provides strong mechanisms for both server-side and client-side caching. It's the only framework I know of to implement all of the server-side recommendations in YSlow. Rails is, in fact, a high-performance framework. If someone says Rails can't scale, ask them what exactly they mean."


So, let me ask you...


If you say Ruby on Rails can't scale, what DO you mean?