Description of problem:

If an user would decide to migrate his engine 3.6 on EL6 to 3.6 on EL7, he would face an issue with "incompatibility" of original engine DB with new postgresql 9.x updated in EL7.

After migration and update all updateable packages (as some are locked via versionlock), the user won't be able to start DB as prerequisite for engine-setup update for rest of engine rpms.

postgresql complains about old version of database and recommends to run 'postgresql-setup upgrade'. Running DB conversion process finishes in changed DB configuration files which engine depends on:

- max connections are set (back) to '100' thus engine-setup would end with error

  solution:
    sed -i 's/^\(max_connections\).*/\1 = 150/;' /var/lib/pgsql/data/postgresql.conf

- DB access control is changed thus engine-setup won't connect to the DB:

  solution:
    cp /var/lib/pgsql/data-old/pg_hba.conf /var/lib/pgsql/data/pg_hba.conf

All this would maybe be worth of document that or to have a ovirt script to prepare DB and its configuration for next engine-setup run.

Version-Release number of selected component (if applicable):
3.6.5

How reproducible:
100%

Steps to Reproduce:
1. engine on el6
2. migrate to el6 via redhat-upgrade-tool (or fedora equivalent)
3. update all rpms, remove obsolete el6 packages
4. try to start DB (fyi some daemons were disabled during migration)
5. engine-setup

Actual results:
undocumented flow, unable to run engine-setup after migration from el6 to el7

Expected results:
either document manual steps or have a ovirt script which would help to "reconfigure" env for next successful engine-setup run

Additional info:
discovered during POC testing of 3.6 el6 - > 4.0 el7