This feature is tied into Cluster Rolling Upgrade. When we are doing a rolling upgrade of a cluster (from WS2012 R2 to vNext), we will temporarily have a mixture of WS2012 R2 hosts and Windows Server vNext hosts. This is referred to as mixed mode.
To avoid issues, the cluster will either:
- Run only features of the down-level hosts
- Run features only on down-level hosts
For example, on a storage cluster (SOFS) running disk deduplication (for VDI personal VMs), the cluster will only run deduplication optimization jobs on the down-level hosts because vNext optimization will make changes that are not backwards compatible.
This mixed mode is TEMPORARY – it should not be run for months or years!!!!
EDIT: Today (17/Dec/2014) I learned that Microsoft will have a support limited of 4 weeks for running a cluster in mixed mode. They want you to finish the migration ASAP, but 4 weeks will be the support limit. There is not hard coded limit, but now you know what they’ll support.
EDIT: I also learned on 17/Dec/2014 that Microsoft will not put a limit on how long v5.0 VMs (VMs with WS2012 R2 host compatibility – more on this later) can run on a higher level cluster.
The goal is that you upgrade each node in the cluster as quickly as possible. You might pause half way through to observe the stability of your hosts (drivers, firmware and external storage/networking hardware). If you’re happy you continue the upgrade. If not, you can rebuild the upgraded hosts to WS2012 R2 and live migrated your VMs back.
Note: VMs will be manually upgraded to vNext and this is why you can live migrate VMs between WS2012 R2 and vNext hosts within the cluster without any issues.
When you have finished the upgrade you will complete the project by raising the cluster functional level. This opens up the cluster’s vNext features and then you should manually upgrade the VMs to v6.0 (vNext).