[ovirt-devel] Change compatibility level for data center/cluster during upgrade - what exactly are we changing?

Hi all, From the upgrade procedure, when we completed most upgrade tasks, we have to change the compatibility level in cluster and data center, and then restart all guest VM afterwards.It seems no down time expected with these 2 changes from the procedure. I wonder what exactly we are changing (a setting in the database/file??) and most important of all, whether there will be a few seconds down time when we made those changes to "data center"? Because I worried if a data center change could lead to storage domain's down time and eventually force hypervisors, guest VM restart before expected time. (My hosts/guest VM directly use HP 3par's storage as boot disks/data disks...) Hope anyone can share any experience on this. Thank you, Patrick Chiang