Welcome to the first post in the Tales from the Field series. In this section I indent to collect the most useful stories about different products and how they are being used in every day situations.

For a starter, I'll cover the new RHV suite, now on version 4.1, and the upgrade path to have an older environment migrated to the latest version.

RHV fundamentals

RHEV is now simply known as Red Hat Virtualization and it has several components:

The RHEV Manager, a system that acts as a central configuration and orchestration point for all hypervisors.
Several types of hypervisors:
- RHEL-6 hypervisors. Used primarily in the RHEV 3.x suite.
- RHEL-7 hypervisors. Used in the latter RHEV 3.x and 4.x suites.
- RHEV-H 6 (vintage) hypervisors. Used primarily in the RHEV 3.x suite.
- RHEV-H 7 (vintage) hypervisors. Used in the latter RHEV 3.x suite.
- RHEV-H 7 (ngn) hypervisors. The new hypervisor in RHEV 4.x suite.

Standard RHEV to RHV upgrade path

Red Hat has put together some documentation and labs that should ease the upgrade process:

The 'RHV upgrade helper' lab
Extensive documentation:
- Upgrading 3.5 to 3.6 :
- Upgrading 3.6 to 4.0 :
  - RHV 4.0 UPGRADE GUIDE
  - How to upgrade RHEV 3.6 Hosted Engine to RHV 4.0
- Upgrading 4.0 to 4.1 :
  - RHV 4.1 UPGRADE GUIDE

The 3.6 to 4.0 step is probably the one that requires most planning, as it involves rebuilding the manager with a new version of the underlying operating system (RHEL6 to RHEL7).

Shortcutting the upgrade

If I'm running a 3.5 cluster I surely do not need to install/upgrade every version of RHEV to end up in the latest version, do I?

Unfortunately, yes. But there are some tweaks that can be done in the process to ease out the migration.

If you are running a 3.5 cluster with either EL6 or EL7 3.5 hypervisors, you'll need to reinstall them. RHV4 uses the next generation node (NGN) hypervisors which are not upgradable from previous versions of RHEV-H. The good news is it's now available a 3.6 NGN version of the hypervisor. So you can reinstall some of your existing 3.5 hosts with 3.6NGN, and that's the only reinstallation you will need.
Once you have updated your manager to 3.5.latest, then to 3.6 you are in a position to start reinstalling your hypervisors with 3.6 NGN. Once that part is done, it is required you change your cluster compatibility settings to 3.6 and recommended you reboot all your VMs. There is an ongoing buzilla BZ#1474870 that should address documenting what is the official recommendation regarding VM reboot on an upgrade scenario.
Once you are in RHEV 3.6 Manager with 3.6 compat level, you need to backup your engine database and keep in a safe place. If you're migrating to a Hosted Engine from a standalone Engine, you'll need it to restore the data. If your plan is to reinstall your standalone Engine with RHEL7, it's the moment to do so.
Once your Engine is available, you'll run a restore then an upgrade so it's upgraded to the latest 4.0 version .
If your 3.6 hypervisors have a configured RHSM/Satellite subscription, you'll be able to launch a 3.6 to 4.0 upgrade from the Manager itself. This greatly eases the upgrade as no manual provision is required.
After all hypervisors have been migrated to 4.0, change cluster compatibility to 4.0 .
Rinse and repeat for Manager & host upgrades to 4.1 .

Happy hacking!