On a system with snapshot support /var should not be part of the root file system, otherwise doing a rollback to a previous state would also roll back the /var contents. On a read-only system this directory has to be writable in any case, variable data is stored inside.

Due to the volatile nature of /var the directory will not be mounted into the new snapshot during the transactional-update run, as this would break atomicity: The currently running system depends on the old state of the data (imagine a database migration was triggered by a package). Any modifications to /var therefore have to be in the new system, i.e. modifying the contents of /var as part of the packaging scripts is not allowed.

The only exception to this rule are directories: Those will be recreated during the first boot into the updated system by the create-dirs-from-rpmdb.service helper service. For all other cases please use one of the options described in Packaging for transactional-updates and Migration / Upgrade in the Packaging guidelines. If a package is breaking this rule by installing files into a directory which is not part of the root file system, then a warning message indicating the affected file is printed at the end of the transactional-update run.