To be clear, most servers are moving to 2012 R2 (most already have) but the machines our SQL clusters run on, specifically, will not.
This is because you have to effectively (or literally) rebuild the Windows cluster from scratch and we just don't get that level of benefit from the 2012 to 2012 R2 upgrade. There are quite a few improvements we care about: native NVMe, better dynamic quorum, better DSC support, better SMB, and such...but not enough to make the upgrade worth it.
Do I get this right? You can not upgrade a server cluster to the next version of the operating system without complete new installation??? Could you please write something about these kind of things, this is very interesting!
I'm not sure a full writeup would have much more detail, but yes. With Windows clustering (on top of which SQL 2012/2014 Availability Groups are based), you can only have a homogenous OS version across the board. Since they all must have the same version, you can't have something like a single 2012 R2 instance in an otherwise 2012 cluster...and since you can't do that, you can't upgrade one at a time. Yay! New cluster time instead!
Trust me, we're bitching about this as are most people and I think changes must be coming there. It doesn't matter how many fancy features you add to the OS if we can't upgrade to it, so they'll have to stop and address that problem.
This is because you have to effectively (or literally) rebuild the Windows cluster from scratch and we just don't get that level of benefit from the 2012 to 2012 R2 upgrade. There are quite a few improvements we care about: native NVMe, better dynamic quorum, better DSC support, better SMB, and such...but not enough to make the upgrade worth it.
Nick Craver - Stack Exchange Sysadmin & Developer