OSTree and /var handling
Default commit/image /var handling
As of OSTree 2024.3, when a commit is “deployed” (queued to boot),
the initial content of /var in a commit will be placed into the
“stateroot” (default var) if the stateroot var is empty.
The semantics of this are intended to match that of Docker “volumes”;
consider that ostree systems have the equivalent of
VOLUME /var
by default.
It is still strongly recommended to use systemd tmpfiles.d snippets
to populate directory structure and the like in /var on firstboot,
because this is more resilent.
Even better, use StateDirectory= for systemd units.
Pitfalls
On subsequent upgrades, normally /var would not be empty anymore
(as it’s typically expected that basics like /var/tmp etc. are created,
if not also other local state such as /var/log etc.). Hence,
no updates from the commit/container will be applied.
To be clear then:
- 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
debs/RPMs which drop files into /opt (i.e. /var/opt)
The default OSTree “strict” layout has /opt be a symlink to /var/opt.
Including any packaged content that “straddles” /usr and /var (i.e. /var/opt)
will over time cause drift because changes in the package will not be reflected on disk.
For situations like this, it’s strongly recommended to enable either
composefs.enabled = true or the root.transient = true option for ostree-prepare-root.conf
and change ensure your commit/container image has /opt as a plain directory. In the former case,
content in /opt will be immutable at runtime, the same as everything else in /usr.
In the latter case content it will be writable but transient.
There’s also a currently-experimental ../man/ostree-state-overlay@.service.xml which can manage stateful writable overlays for individual mounts.
Apache default content in /var/www/html
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”.
/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.
Previous ostree /var and tmpfiles.d /usr/share/factory/var
From OSTree versions 2023.8 to v2024.3 the /usr/lib/tmpfiles.d/ostree-tmpfiles.conf file included this snippet:
# 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 - - - - -
Until version 0.13.2 of the ostree-ext project, content in /var in fetched container images is moved to /usr/share/factory/var, but this no longer happens when targeting ostree v2024.3.
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.
This has been reverted, and the semantics defer to the above ostree semantic.