You've partly got the wrong of the stick. YARV is not a mongrel replacement, it's the name of a virtual machine for ruby, effectively ruby 1.9 is yarv. What you would need to do is get ruby 1.9. You would then still run mongrel on top of that. I've no idea whether rails currently runs on 1.9 ( I would guess that it's probably not 100% there, since I've seen quite a few commits go past with messages to the effect of 'fixed for ruby 1.9 compatibility')
Fred