docs: Add var.md

This one overlaps a bit with some other sections...the docs
need a bigger rework, but this is better than we had before.

docs/adapting-existing.md

@@ -69,6 +69,10 @@ d /var/mnt 0755 root root -
 d /run/media 0755 root root -
 ```
 
+However, as of OSTree 2023.9 there is support for a `root.transient`
+model, which can increase compatibility in some scenarios.  For more
+information, see `man ostree-prepare-root.conf`.
+
 Particularly note here the double indirection of `/home`.  By default,
 each deployment will share the global toplevel `/home` directory on
 the physical root filesystem.  It is then up to higher levels of

docs/var.md

@@ -0,0 +1,60 @@
+---
+nav_order: 6
+---
+
+# OSTree and /var handling
+
+{: .no_toc }
+
+1. TOC
+{:toc}
+
+As of OSTree 2023.8, the `/usr/lib/tmpfiles.d/ostree-tmpfiles.conf` file gained this snippet:
+
+```text
+# Automatically propagate all /var content from /usr/share/factory/var;
+# the ostree-container stack is being changed to do this, and we want to
+# encourage ostree use cases in general to follow this pattern.
+C+! /var - - - - -
+```
+
+This is inert by default.  However, there is a pending change in the ostree-container stack which will move all files in `/var` from fetched container images into `/usr/share/factory/var`.  And other projects in the ostree ecosystem are now recommended do this by default.
+
+Together, this will have the semantic that on OS updates, on the next boot (early in boot), any new files/directories will be copied.  For more information on this, see [`man tmpfiles.d`](https://man7.org/linux/man-pages/man5/tmpfiles.d.5.html).
+
+However, `tmpfiles.d` is not a package system:
+
+## Pitfalls
+
+- Large amounts of data will slow down firstboot while the content is copied (though reflinks are used if available)
+- Any files which already exist will *not* be updated.
+- Any files which are deleted in the new version will not be deleted on existing systems.
+
+## Examples
+
+### Apache default content in `/var/www/html`
+
+The `tmpfiles.d` model may work OK for use cases that wants to treat this content as locally mutable state.  But in general, such static content would much better live in `/usr` - or even better, in an application container.
+
+### User home directories and databases
+
+The semantics here are likely OK for the use case of "default users".
+
+### debs/RPMs which drop files into `/opt` (i.e. `/var/opt`)
+
+The default OSTree "strict" layout has `/opt` be a symlink to `/var/opt`.
+However, `tmpfiles.d` is not a package system, and so over time these will slowly
+break because changes in the package will not be reflected on disk.
+
+For situations like this, it's recommended to enable the `root.transient = true` option for `ostree-prepare-root.conf`
+and change your build system to make `/opt` a plain directory.
+
+### `/var/lib/containers`
+
+Pulling container images into OSTree commits like this would be a bad idea; similar problems as RPM content.
+
+### dnf `/var/lib/dnf/history.sqlite`
+
+For $reasons dnf has its own database for state distinct from the RPM database, which on rpm-ostree systems is in `/usr/share/rpm` (under the read-only bind mount, managed by OS updates).
+
+In an image/container-oriented flow, we don't really care about this database which mainly holds things like "was this package user installed".  This data could move to `/usr`.