[ovirt-users] Engine migration and host import

[Ben Bradley]

Hi All

I've been running a single-host ovirt setup for several months, having 
previously used a basic QEMU/KVM for a few years in lab environments.

I currently have the ovirt engine running at the bare-metal level, with 
the box also acting as the single host. I am also running this with 
local storage.

I now have an extra host I can use and would like to migrate to a hosted 
engine. The following documentation appears to be perfect and pretty 
clear about the steps involved: 
https://www.ovirt.org/develop/developer-guide/engine/migrate-to-hosted-engine/ 
and 
https://www.ovirt.org/documentation/self-hosted/chap-Migrating_from_Bare_Metal_to_an_EL-Based_Self-Hosted_Environment

However I'd like to try and get a bit more of an understanding of the 
process that happens behind the scenes during the cut-over from one 
engine to a new/hosted engine.

As an experiment I attempted the following:
- created a new VM within my current environment (bare-metal engine)
- creating an engine-backup
- stopped the bare-metal engine
- restored the backup into the new VM
- ran engine-setup within the new VM
The new engine started up ok and I was able to connect and login to the 
web UI. However my host was "unresponsive" and I was unable to manage it 
in any way from the VM. I shut the VM down and started the bare-metal 
ovirt-engine again on the host and everything worked as before. I didn't 
try very hard to make it work however.

The magic missing from the basic process I tried is the synchronising 
and importing of the existing host, which is what the hosted-engine 
utility does.

Can anyone describe that process in a bit more detail?
Is it possible to perform any part of that process manually?

I'm planning to expand my lab and dev environments so for me it's 
important to discover the following...
- That I'm able to reverse the process back to bare-metal engine if I 
ever need/want to
- That I can setup a new VM or host with nothing more than an 
engine-backup but still be able to regain control of exiting hosts and 
VMs within the cluster

My main concern after my basic attempt at a "restore/migration" above is 
that I might not be able to re-import/sync an existing host after I have 
restored engine from a backup.

I have been able to export VMs to storage, remove them from ovirt, 
re-install engine and restore, then import VMs from the export domain. 
That all worked fine. But it involved shutting down all VMs and removing 
their definitions from the environment.

Are there any pre-requisites to being able to re-import an existing 
running host (and VMs), such as placing ALL hosts into maintenance mode 
and shutting down any VMs first?

Any insight into host recovery/import/sync processes and steps will be 
greatly appreciated.

Best regards
Ben