For the record, I obviously support this change.  Responding to a few threads:

On Wed, Dec 29, 2021, at 10:16 AM, Peter Robinson wrote:

> How does this work on RO /usr files systems? I thought data in /usr
> was supposed to be static/ It works for rpm-ostree because it's
> updated at tree creation time.

I think you know this but it's worth elaborating on here; rpm-ostree supports 
client-side layering (and overrides too) and even live updates - and those all 
operate by default while maintaining `/usr` read-only from the perspective of 
the rest of the system.

The way this works ultimately is quite simple; the underlying filesystem is 
writable, we just remount it writable *in a private mount namespace*.  So even 
when you do e.g. `rpm-ostree install -A usbguard`, there is no point where 
*other* processes can write.  

People are focusing a bit too much on "read-only" in this thread - it's more 
about "lifecycle binding" and versioning the binaries together with metadata 
about the binaries.

On Thu, Dec 30, 2021, at 1:57 PM, Chris Murphy wrote:

>> What happens if /var/lib is read-only? Changing (fixing?) this would
>> be a pre-requisite to this proposal, we don't want 'dnf list' to break.

Why would /var/lib be read-only, but /usr be writable?

> Why should it be a prerequisite? In all Fedora editions and spins with
> dnf, /usr and /var are read-write. In the case of rpm-ostree based
> editions and spins, they don't include dnf. 

Remember rpm-ostree links to libdnf, and significant code is hence shared.
That's part of the ASCII-art architecture diagram in the docs
https://coreos.github.io/rpm-ostree/

The way I'd say this is it aligns "traditional" dnf systems with what 
rpm-ostree has been doing for many years now, and that will help share *even 
more* code and logic in the future.
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