Here is a blog highlighting some issues when migrating eins.de from a php application to a ruby on rails application. The old php site was able to handle 900,000 page impressions before having problems. Their first day on Ruby on Rails was failing after 150,000 page impressions, however after tweaking (Can you call changing applications and server rols tweaking?) they eventually were left with an application that could easily handle 1,200,000 page impressions without crashing.
The old codebase roughly consisted of around 50.000 lines of PHP code (plus a closed-source CMS that’s not included in this calculation). We’ve rewritten most of it (some features were left out on purpose) in about 5.000 lines of Rails code.
Update March 18: A follow-up article addressing reader comments is now available here.
Update March 20: Stage 2 is online.
Update March 27: Stage 3 is online.
Update April 03: Stage 4 is online