Proposal: Version-specific CentOS-release-xen-NN Packages

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Thank you for those who raised the issue of upgrading across potentially disruptive major versions (like the Xen 4.4 -> 4.6
update), and for everyone who weighed in.

After discussing options with users here on this list (as well as in the IRC channel), further discussing things in the Virt SIG IRC
meeting, and also raising the issue with other SIGs on the CentOS-devel list, here are my thoughts. (Skip down to the “Proposal”
sections for the TLDR version.)

= The issue To summarize the issue:

* Some users want to get whatever the latest supported version of Xen automatically via yum, without having to do anything but a “yum update”. They want “yum update” to give them a new major version when one is available. We can call this group “auto-upgrade”, because they expect a major version upgrade to happen automatically.

* Other users want “yum update” to have fairly strong guarantees not to break their systems — and thus want “yum update” *not* to give them a new major version, but want to have to manually do something specific (such as installing a different package) to get the updated version. Or at very least, they expect not to get a new major version that is expected to break things. We can call this group
“manual-upgrade”, because they want to manually upgrade major versions.

* At the moment, the Virt SIG is only committed to making security updates to one version of Xen at a time

* The 4.4 -> 4.6 update may be particularly disruptive for some users, since xend/xm will no longer be available.

* Many users may not follow this mailing list, so may not know about the impending update

* There are new XSAs coming out in 2 days (17 February).

There are basically two “problems” to solve.

The first thing to look at is what we would like things to look long-term, given that some users want auto-upgrade and some users want manual-upgrade.

The second thing to look at is what to do in the current situation, given that we have users from both camps with the same set of packages installed, who may not read this mailing list.

A couple of goals (some of which are on conflict):

* We’d like to support both types of users

* We want to minimize breakage for people who do don’t follow this list, and do “yum update” without noticing the major upgrade that gets pushed out.

* We want to minimize the security exposure for people who don’t follow this list, and who don’t notice that there haven’t been any updates in response to XSAs.

Additionally, we’d like if possible to be able to make it possible for anyone who wants to maintain older versions of Xen to do so in parallel with the newer versions.

= Proposal: Long term Going forward, this is what I propose we aim for:

* Have separate repos for each version of Xen; at the present, this means xen-44 and xen-46.

* Have version-specific CentOS-release-xen-NN pakcages which will always point to that version of the repo. So CentOS-release-xen-44
will point to xen-44, CentOS-release-xen-46 will point to xen-46. Users in the “manual-upgrade” camp can install one of these versions, knowing that a simple “yum update” will never move them across versions. When they’re ready to upgrade, they can just install the newer version; yum will choose the newest version from all the repos available to it.

* Have a generic CentOS-release-xen package which will always point to the most recent ‘released’ Virt SIG version of Xen. Users in the
“auto-upgrade” camp can install this version, knowing that they’ll always get the version with the latest security fixes.

This will also make it simple for community members to step up and support older versions if they desire.

= Discussion: Short term For those who do not follow the list, we have to essentially choose one update policy for them. Will the current users expecting
“manual-upgrade” have “auto-upgrade” imposed on them (potentially breaking on update)? Or will the current users expecting
“auto-upgrade” have “manual-upgrade” imposed on them (potentially missing important security updates)?

For someone to be negatively affected by the “manual-upgrade” choice, they only have to be running Xen in any configuration.

For someone to be negatively affected by the “auto-upgrade” choice, they have to
* still be running xend (in spite of having a warning printed on every xm command invocation for the last 18 months)
* update their systems without checking / noticing the new major version

Even then, they should have the option of downgrading with “yum downgrade”; there should only be major carnage if they automatically update large numbers of servers at once.

When I brought this question up with the other SIGs, the general sentiment was that, as community-run projects, we do not have nearly the testing effort required to make it possible for users to simply run “yum update” without checking first.

So on the whole, I think the aggregate risk is minimized if we go with the “auto-upgrade” option for those who don’t specifically make a choice.

= Proposal: Short term
* I’ll try to get the new xen-NN repos, along with CentOS-release-xen-NN packages up and working tomorrow (Tuesday)

* Wednesday I’ll push updates for the 4.6 packages to the xen-46 repos.

* CentOS-release-xen will point to the xen-44 repos for two weeks. During that time, those who want the XSA updates will have to install CentOS-release-xen-46. Those who don’t want to update to 4.6 should install the CentOS-release-xen-44 package *and remove* the CentOS-release-xen package.

* After those two weeks, CentOS-release-xen will switch to the xen-46
repos, giving the XSA updates to everyone who hasn’t yet updated.

I know this doesn’t make everyone happy, but I’ve tried to make the best of a tricky situation. Thank you all for the input you’ve given.

-George

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