This is the probably the most common setup. In this case, two root partitions of the same size are used (often called “A” and “B”). When running from “A”, an update is installed into “B” and vice versa. Both slots are intended to contain equivalent software, including the main application.
To reduce complexity, the kernel and other files necessary for booting the system (such as the device tree) are stored in the root-fs partition (usually in /boot). This requires a boot-loader with support for the root-fs type.
system.conf would contain two slots similar to the following:
The main advantage of this setup is its simplicity:
An update can be started when running in either slot and while the main application is still active.
The fallback logic in the boot-loader can be relatively simple.
Easy to understand update process for end-users and technicians.
The main reasons for not using it are either:
Too limited storage space (use asymmetric slots instead)
Additional requirements regarding redundancy or update flexibility (see below)
This setup is useful if the storage space is very limited. Instead of requiring two partitions each large enough for the full installation, a small partition is used instead of the second one (often called “main” and “update” or “rescue”).
The slot configuration for this in
system.conf could look like this:
To update the main system, a reboot into the update system is needed (as otherwise the main slot would still be active). Then, the update system would trigger the installation into the main slot and finally switch back to the newly updated main system. The update system itself can be updated directly from the running main system.
Some disadvantages of this configuration are:
Two reboots are required for an update.
A failed update results in an unavailable main application until a subsequent update is installed successfully.
If some data in the main slot needs to be preserved during the update, it must be stored somewhere else before writing the new image to the slot and then restored.
As the update system is normally small enough to fit completely into RAM, it can
be stored as a Linux kernel with internal initramfs.
This avoids compressing kernel and user-space separately, increasing the
For this, the update slot type should be configured to
Splitting a system into multiple slots can be useful if the application should be updated independently of the base system. This can be combined with either symmetric or asymmetric setups as described above.
For example, the main application could be split of from the root file-system. This can be useful if the base system is developed independently from the application(s) or by a different team. By explicitly distinguishing between the two, different versions of the application or even completely different applications can reuse the same base system (root-file-system).
Another reason to configure multiple slots for one system can be to store the boot files (kernel, …) separately, which can help reduce boot time and complexity in the boot-loader.
Currently, RAUC has no way to ensure compatibility between rootfs and appfs when installing a bundle containing only an image for one of them. Either always build bundles containing images for all required slots or ensure that incompatible updates are not installed outside of RAUC. To solve this, a bundle would need to contain the metadata (size and hash) for the missing bundle and RAUC would need to verify the state of those slots before installing the bundle.
By adding an additional rescue (or recovery) slot to one of the symmetric scenarios above, the robustness against some error cases can be improved:
A software error has remained undetected over some releases, rendering both normal slots inoperable over time.
The normal slots are mounted read-write during normal operation and have become corrupted (for example by incorrect handling of sudden power failures).
A configuration error causes both normal slots to fail in the same way.
The rescue slot would not be changed by normal updates (which only write to A and B in turn). Depending on the use case, the boot-loader would start the rescue system after repeated boot failures of the normal systems or on user request.