Symptoms
Upon using "yum history undo" on Virtuozzo Hybrid Server 7.5 Update 6 node per Downgrading to Older Versions page, there are multiple errors on downgrade operation:
# yum history undo {transaction_ID}
--snip---
Failed to downgrade: prl-disp-service-7.0.1261.5-1.vz7.x86_64
Failed to downgrade: prlctl-7.0.248.1-1.vz7.x86_64
And as the result of such error there is no prl-disp unit running and packages related to prl-disp are getting removed:
]# systemctl status prl-disp
Unit prl-disp.service could not be found.
Cause
The issue is recognized as product bugs PSBM-153291 and PSBM-153297 and will be fixed in future versions of Virtuozzo Hybrid Server 7.5
Resolution
Install the removed packages manually to workaround the issue.