[Rpm-maint] [rpm-software-management/rpm] RFE: container-native rpmdb format (Issue #2005)

[Colin Walters]

The way OCI/Docker containers work is a [series of layers](https://github.com/opencontainers/image-spec/blob/main/layer.md).  In overlayfs, modified files are "copied up".

There are two issues:

 - rpms have a lot of metadata, so the rpmdb can be of nontrivial size
 - The current main rpmdb format (sqlite) being a single big file means that it gets duplicated in each derived image

In the common case of something like e.g.

```
FROM registry.fedoraproject.org/fedora:35
RUN dnf -y install cowsay && dnf clean all
```

The resulting tar layer from the `RUN` command has an *entirely duplicated* copy of the rpm database, just with `cowsay` and its dependencies added.

It's quite common for builds like this to actually form the base image for further images - and this duplication gets compounded.

A strawman proposal here is something like `/usr/lib/sysimage/rpmdb.d` with something simple like zstd-compressed JSON files storing data instead.  The files could be named something simple like `0000.json.zstd` and then later changes which add packages add just a new `0001.json.zstd` file or so.  Or, we could keep sqlite and union those; I don't have a really strong opinion.  (Well, not really "union" literally but more "merge", since we should support removing or upgrading packages from prior layers)



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