We have no such environment. It is expensive, we can't afford this for now.
We do have QA cluster, but before even trying the 1.1.0 -> 1.1.9 / 1.2.1 upgrade on it (we were a bit undecided about the version ;-) ), I did some experiments using ccm ( https://github.com/pcmanus/ccm ) on my workstation with a < 0.01% sample of productin data which made me aware of some big problems I didn't expected. OK, there were two more problems I discovered later, on real cluster, but they were minor issues that didn't affect cluster availability - all the major problems occured and were solved during my "local" tests.
If test cluster is too expensive (but isn't data loss expensive too?) I strongly encourage you, at least, to do some tests using ccm :-)
M.
