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docs: Add var.md

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This one overlaps a bit with some other sections...the docs
need a bigger rework, but this is better than we had before.
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Colin Walters 2023-12-08 14:01:13 -05:00
parent a0f7b3c789
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@ -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

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---
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`.