[ovirt-devel] Understanding Cluster / Datacenter upgrades

[Daniel Helgenberger]

Hello,

I seem to have some issues understating what and when something happens 
if I change a cluster or datacenter to a higher version after upgrading 
engine.
Maybe somebody could help me understand how this works in general and 
when features will get enabled? If possible, pinpoint it to concrete 
features added in 3.5.

My assumptions learned form experience so far, upgrading oVirt 3.4 to 
3.5; please correct if wrong:
- New features are enabled if I set the cluster compat version to a 
newer version after upgrade
- Prior to upgrading a datacenter all cluster need to run the higher 
version (seems logical).
- This involves Engine database updates (?) and, more crucial, it will 
change settings and data on the storage domains.
- Because of that, downgrade is not possible any more
- Also, its usually safer to keep the old cluster version while using a 
newer engine version; because it is more likely to run into regressions 
with an upgraded datacenter (see regression blockers for oVirt 3.5.1).

My questions so far:
- What happens if I only upgrade the cluster and *not* yet the datacenter?
- In case of 3.5; exactly when could I have used the new NUMA 
management? I assume after upgrading the cluster?
- In case of 3.5; exactly when could I have used the new QoS settings 
(like disk profiles) - here I assume after upgrading the datacenter?

If the the case of NUMA policy is not true but happens only after 
upgrading the datacenter, what is the purpose of the cluster compat 
version setting? I can think of making sure a certain minimum VDSM 
version is running?

Thanks a million,

-- 
Daniel Helgenberger
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