[ovirt-devel] Relocating the vdsm upgrade logic into a new or existing service at boot time

[Fabian Deutsch]

Hey,

the logic which is run on vdsm upgrades is located in the specfile, and thus run
when the rpm get's an update.
This is fine as long as rpm is involved (like when rpm or yum or dnf is used), but it
becomes - and already is - an issue for flows where the upgrade happens without
rpm, like on oVirt Node.

On oVirt Node the image for booting the host is getting replaced, we currently mimic
the upgrade process of the specfile. But this error prone - we had a lot of bugs around
this area (remember the problems with the hooks?), and we need to watch out for changes
in the upgrade logic.

To simplify and especially unify the logic between Node and normal hosts, I'd like to
discuss if the upgrade logic can be moved to a separate service or included in one of the
existsing services.

The simplest approach to me would be top create a new service which is started before
libvirtd and vdsmd/supervdsmd to run some logic if an upgrade of vdsm is detected.
This habit - to do the upgrade logic during boot - is actually not new and systemd
provides some helpers (ConditionNeedsUpdate=) around this to ease the implementation.

Thoughts?

Greetings
- fabian


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[1] http://0pointer.net/blog/projects/stateless.html
[2] http://www.freedesktop.org/software/systemd/man/systemd.unit.html