docs: Add some description of container storage

Signed-off-by: Colin Walters <[email protected]>

Author: cgwalters

docs/src/SUMMARY.md

@@ -39,6 +39,7 @@
 - [Image layout](bootc-images.md)
 - [Filesystem](filesystem.md)
 - [Filesystem: sysroot](filesystem-sysroot.md)
+- [Container storage](filesystem-storage.md)
 
 # More information
 

docs/src/filesystem-storage.md

@@ -0,0 +1,90 @@
+# Container storage
+
+The bootc project uses [ostree](https://github.com/ostreedev/ostree/) and specifically
+the [ostree-rs-ext](https://github.com/ostreedev/ostree-rs-ext/) Rust library
+which handles storage of container images on top of an ostree-based system.
+
+## Architecture
+
+```mermaid
+flowchart TD
+    bootc --- ostree-rs-ext --- ostree-rs --- ostree
+    ostree-rs-ext --- containers-image-proxy-rs --- skopeo --- containers/image
+```
+
+There were two high level goals that drove the design of the current system
+architecture:
+
+- Support seamless in-place migrations from existing ostree systems
+- Avoid requiring deep changes to the podman stack
+
+A simple way to explain the current architecture is that podman uses
+two Go libraries:
+
+- https://github.com/containers/image
+- https://github.com/containers/storage/
+
+Whereas ostree uses a custom container storage, not `containers/storage`.
+
+## Mapping container images to ostree
+
+[OCI images](https://github.com/opencontainers/image-spec) are effectively
+just a standardized format of tarballs wrapped with JSON - specifically
+"layers" of tarballs.
+
+The ostree-rs-ext project maps layers to OSTree commits.  Each layer
+is stored separately, under an ostree "ref" (like a git branch)
+under the `ostree/container/` namespace:
+
+```
+$ ostree refs ostree/container
+```
+
+### Layers
+
+The `ostree/container/blob` namespace tracks storage of a container layer
+identified by its blob ID (sha256 digest).
+
+### Images
+
+At the current time, ostree always boots into a "flattened" filesystem
+tree.  This is generated as both a hardlinked checkout as well as
+a composefs image.
+
+The flattened tree is constructed and committed into the 
+`ostree/container/image` namespace.  The commit metadata also includes
+the OCI manifest and config objects.
+
+This is implmented in the [ostree-rs-ext/container module](https://docs.rs/ostree-ext/latest/ostree_ext/container/index.html).
+
+### SELinux labeling
+
+A major wrinkle is supporting SELinux labeling.  The labeling configuration
+is defined as regular expressions included in `/etc/selinux/$policy/contexts/`.
+
+The current implementation relies on the fact that SELinux labels for
+base images were pre-computed.  The first step is to check out the "ostree base"
+layers for the base image.
+
+All derived layers have labels computed from the base image policy.  This
+causes a known bug where derived layers can't include custom policy:
+<https://github.com/ostreedev/ostree-rs-ext/issues/510>
+
+### Origin files
+
+ostree has the concept of an `origin` file which defines the source
+of truth for upgrades.  The container image reference for each deployment
+is included in its origin.
+
+## Booting
+
+A core aspect of this entire design is that once a container image is
+fetched into the ostree storage, from there on it just appears as
+an "ostree commit", and so all code built on top can work with it.
+
+For example, the `ostree-prepare-root.service` which runs in
+the initramfs is currently agnostic to whether the filesystem tree originated
+from an OCI image or some other mechanism; it just targets a
+prepared flattened filesystem tree.
+
+This is what is referenced by the `ostree=` kernel commandline.