[CentOS] https://blog.centos.org/2020/12/future-is-centos-stream/

Wed Dec 9 15:13:39 UTC 2020
Phil Perry <pperry at elrepo.org>

On 09/12/2020 03:26, Brendan Conoboy wrote:
> On Tue, Dec 8, 2020 at 4:19 PM Pete Biggs <pete at biggs.org.uk> wrote:
>> On Tue, 2020-12-08 at 17:54 -0500, Matthew Miller wrote:
>>> On Tue, Dec 08, 2020 at 03:15:17PM +0000, Pete Biggs wrote:
>>>> "CentOS will become the developer playground"
>>> This one is categorically not the case. Even Fedora isn't a developer
>>> playground. Everything landing in CentOS Stream is actually *planned*
>> (with
>>> emphasis intentional) to go in a future RHEL release.
>> It's all the talk of SIGs and developing and testing and that Stream
>> will be the centerpiece of that. That's what I meant.
> I don't know if I'd call SIGs a playground, but they're certainly an
> important venue for communities to explore variations.
>>> Previously, all the development around RHEL releases was done in secret,
>> in
>>> the Red Hat black box. Now it's out of the box and can be watched. There
>> may
>>> be some launch pains, but I expect the average quality of an update
>> hitting
>>> CentOS Stream to be very high.
>> I don't get that from the documents released today.  If Stream is *not*
>> a test-bed, then surely the code that appears in Stream must be fully
>> formed in secret behind the scenes first. Yes, it will appear piecemeal
>> rather than in one big chunk, but it has been categorically denied that
>> Stream is not a RHEL 8.n+1 beta and is more a RHEL 8.n+1 RC/rolling
>> release.
> I think maybe some of the nervousness about CentOS Stream comes from RHEL
> seeming opacity on its development model.  As one of the architects of our
> development process I'd be happy to field questions.  I'll start by making
> a two points in case they're in any way unclear:
> 1. Everything that goes into RHEL lands upstream first, then the patches
> are backported into the RHEL releases.
> 2. Most of the work we do or plan on doing is in bugzilla.redhat.com.  If
> you go into the RHEL8 product queue today and file a bug you'll see "CentOS
> Stream" as a "Version" where an issue is encountered.
> I think what a lot of people are concerned about is the rolling-release
>> aspect of this. There will be no definitive versioning of CentOS in the
>> future - all you will be able to say is "fully updated" and it won't be
>> possible to slot a CentOS system in to exactly match a RHEL version.
>> Will third party RPMs built against RHEL 8.x be installable on a CentOS
>> 8 Stream system? The answer is surely "it depends", but there are a lot
>> of hardware vendors that target drivers to RHEL releases, which may
>> well make CentOS non-viable for hardware that doesn't have drivers
>> built in to the kernel.
> Generally if they follow the ABI guidelines I would expect it to work.
> Those are here: https://access.redhat.com/articles/rhel8-abi-compatibility
> For loadable kernel modules there's a kernel ABI.

Hi Brendan,

This point is *critical*, so I apologise in advance for the lengthy 
post, *you* are breaking the kernel ABI between RHEL8 and Stream.

One of the main unique selling points of RHEL is the stability of it's 
kernel ABI. No other distro provides this. The very nature of rolling 
kernel updates in Stream breaks the kernel ABI and breaks compatibility 
between RHEL8 and Stream. What works on RHEL8 may not work on Stream. At 
the kernel level, the two products diverge in fundamental compatibility 
and are not compatible, are no longer the same.

How bad is this divergence / breakage? Well, we know the kernel ABI will 
change from time to time, almost exclusively at new point releases due 
to the massive backporting work that goes into the RHEL kernel. And this 
is fine, we know it's coming, we know when it's coming, and we can plan 
for it's impact. It's a price well worth paying.

To put this in context, at elrepo I currently help maintain around 50 
3rd party kernel driver packages for RHEL8. When RH released RHEL8.3, 
every single package in our repository broke due to changes in the 
kernel ABI in the 6 month period between RHEL8.2 and RHEL8.3. It's not 
ideal, but we accept it as a price we pay for the otherwise excellent 
stability of the kernel ABI during the proceeding 6 months. As I said 
above, we know it's coming, we know when it's coming, and we can plan 
for it.

Now contrast that with Stream. Every kernel update in Stream has the 
potential to break the kernel ABI causing packages built for RHEL to 
break. We don't know when that may happen (only that it will), we don't 
know how often it will happen, we have no idea which packages it will 
break. and we have no way to fix it. Consequently, elrepo has been 
unable to support Stream kernels.

It is not just elrepo's users that the Stream kernels will affect. All 
OEM hardware manufacturers releasing 3rd party driver rpms as part of 
Red Hat's Driver Update Programme or otherwise will be similarly 
affected, and their driver updates will not be applicable to or 
compatible with CentOS Stream. In fact, RHEL's own driver update 
packages will likely need rebuilding against each Stream kernel update, 
although presumably you are in the unique position of being able to 
integrate any changes directly into the Stream kernel negating the need 
for driver updates and to mitigating the impact to yourselves.

Luckily the solution is simple - do not include rolling beta kernel 
updates as part of Stream - rather just ship regular RHEL8 kernel 
updates that retain kernel ABI compatibility rather than breaking 
compatibility of the Stream product. I see no compelling reason to ship 
rolling kernel updates to Stream - it's not like I will ever be able to 
commit a patch and have you accept/merge it, and it's totally useless 
for me as a developer as I can't develop anything against a constantly 
moving target, and anyone who does require access has always had that 
available through their Red Hat OEM Partner/Developer accounts. Shipping 
rolling kernel updates in Stream adds no value and simply breaks 
compatibility between RHEL8 and Stream at the most fundamental level. If 
you are able to retain kernel ABI compatibility between RHEL8 and Stream 
kernels, then we (and other OEMs) will be able to continue to support 
Stream users, otherwise Stream users will have to look to alternative