coreos: forced downgrade doc

[philips]

This is an "emergency" procedure that requires root but can be done:

Edit /etc/coreos/update.conf to the desired channel.

cp /usr/share/coreos/release /tmp
mount -o bind /tmp/release /usr/share/coreos/release

Edit /tmp/release and set the version to 0.0.0.

systemctl restart update-engine
update_engine_client -update

Your system will reboot onto the downgraded channel.

cc @polvi @marineam @joshix

[philips]

@omkensey or @joshix or someone else can you take on this guide and append it to the rollback instructions? https://coreos.com/os/docs/latest/manual-rollbacks.html

Thank You!

[philips]

cc @brianredbeard

[marineam]

You can add the release version directly to update.conf, no need for the bind mount. Just remember to remove it afterwards so you don't endlessly upgrade.

[joshix]

@marineam I think the bind mount makes it more convenient. You don't have to remember to reset anything after the reboot. Let me know if you think that's awful. See #818.

/cc @philips

[remoe]

I want to downgrade from 1235.9.0 to 1185.5.0. Is it possible to specify a version? The docs are not clear about this.

[crawford]

It is not possible to downgrade. You can rollback, but that is just to the previously-installed version. What can we change in the wording to make this more clear?

[remoe]

ok, but i don't find a clear statement in https://coreos.com/os/docs/latest/manual-rollbacks.html about this. For example: It is not possible to downgrade to a specific version of CoreOS :)

[crawford]

In https://coreos.com/os/docs/latest/manual-rollbacks.html#performing-a-manual-rollback, we have the following:

...we can tweak the process so that it boots an older image (assuming it's still intact on the passive partition).

There isn't much under https://coreos.com/os/docs/latest/manual-rollbacks.html#forcing-a-channel-downgrade though. We can clarify how this works.