<html><body><div style="font-family: arial, helvetica, sans-serif; font-size: 12pt; color: #000000"><div>Hello,<br></div><div><br data-mce-bogus="1"></div><div>We had a rather unpleseant surprise while upgrading our production datacenters to oVirt 4.0 => 4.1 (although I think this problem is not related to oVirt 4.1). </div><div>Some critical VMs simply refused to reboot without any particular error message other than "VM failed to boot"<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div>After a stressfull 2h of investigation, it turns out that the problematic VMs shared the same "exotic" condition. They were not rebooted since the latest major upgrade 3.6 => 4.0 (I know, this is not a good practise and we underestimated it's priority, we will change our reboot policies)<br data-mce-bogus="1"></div><div>Their "custom compatilibity setting" was set to 3.6 !! When changing this setting to "Empty", The VM booted normaly. Hard to find, easy to fix !! ;)<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div>Neverthess, there is little to none information/documentation about the usefullness of this setting, and I have a hard time understanding the consequences of changing it manually.<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div>Our oVirt datacenter (which has engine 4.1.1, but some hosts on 4.0, so it's still on 4.0 compatibility) hosts currently +- 200 Vms, </div><div>- 10 with "custom compatilibity setting" set to 3.6 - which will not (reb)boot in current state, </div><div>- 30some with "custom compatilibity setting" set to 4.0 - which (re)boot normally <br data-mce-bogus="1"></div><div>- the rest with "custom compatilibity setting" set to "empty" - which (re)boot normally <br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div>So can a oVirt guru please answer the following questions :<br data-mce-bogus="1"></div><div><br></div><div>- What does this setting do? which are the consequences of changing it manually ?<br data-mce-bogus="1"></div><div>- Is it "normal" that a VM not reboooted since the 3.6 update does not boot if a new major upgrade is done on hostig host ? (maybe a exotic bug worth correcting ?)<br data-mce-bogus="1"></div><div>- Which are the possible consequences of manually setting this setting to "Empty" in all my VMs (running or stopped) ?<br data-mce-bogus="1"></div><div>- Which events will change this setting automatically ? (cluster major version upgrade, first reboot after upgrade, ...) ?<br data-mce-bogus="1"></div><div>- Some of my VMs have custom_compatibility_version set to 4.0 (in REST API) eventough they have been recently rebooted and "custom compatilibity setting" is empty in GUI, who's this possible ?<br data-mce-bogus="1"></div><div><br data-mce-bogus="1"></div><div data-marker="__SIG_PRE__"><div>cordialement, regards,</div><div><br data-mce-bogus="1"></div><table style="margin: 0px; table-layout: fixed; border: none; width: 345pt; padding: 0px;" data-mce-style="margin: 0px; table-layout: fixed; border: none; width: 345pt; padding: 0px;" width="460" cellspacing="0" cellpadding="0" border="0"><tbody><tr><td style="margin: 0; font-size: 11px; font-family: Arial,sans-serif; color: #2b3c7a; line-height: 16px; width: 86.25pt; text-align: left;" data-mce-style="margin: 0; font-size: 11px; font-family: Arial,sans-serif; color: #2b3c7a; line-height: 16px; width: 86.25pt; text-align: left;" width="115" valign="top"><a href="https://www.lyra-network.com/" style="border: none; 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