2. RAUC Basics¶
From a top view, the RAUC update framework provides a solution for four basic tasks:
- generating update artifacts
- signing and verification of update artifacts
- robust installation handling
- interfacing with the boot process
RAUC is basically an image-based updater, i.e. it installs file images on devices or partitions. But, for target devices that can have a file system, it also supports installing contents from tar archives. This often provides much more flexibility as a tar does not have to fit a specific partition size or type. RAUC ensures that the target file system will be set up correctly before unpacking the archive.
2.1. Update Artifacts – Bundles¶
In order to know how to pack multiple file system images, properly handle installation, being able to check system compatibility and for other meta-information RAUC uses a well-defined update artifact format, simply referred to as bundles in the following.
A RAUC bundle consists of the file system image(s) or archive(s) to be installed on the system, a manifest that lists the images to install and contains options and meta-information, and possible scripts to run before, during or after installation. A bundle may also contain files not referenced in the manifest, such as scripts or archives that are referenced by files that are included in the manifest.
To pack this all together, the default bundle format uses SquashFS. This provides good compression while allowing to mount the bundle without having to unpack it on the target system. This way, no additional intermediate storage is required.
A key design decision of RAUC is that signing a bundle is mandatory. For development purpose a self-signed certificate might be sufficient, for production the signing process should be integrated with your PKI infrastructure.
A RAUC Bundle should always unambiguously describe the intended target state of the entire system.
2.2. RAUC’s System View¶
Apart from bundle signing and verification, the main task of RAUC is to ensure that all images in your update bundle are copied in the proper way to the proper target device / partition on your board.
In order to allow RAUC to handle your device right, we need to give it the right view on your system.
In RAUC, everything that can be updated is a slot. Thus a slot can either be a full device, a partition, a volume or simply a file.
To let RAUC know which slots exists on the board that should be handled, the slots must be configured in a system configuration file. This file is the central instance that tells RAUC how to handle the board, which bootloader to use, which custom scripts to execute, etc.
The slot description names, for example, the file path the slot can be accessed with, the type of storage or filesystem to use, its identification from the bootloader, etc.
2.4. Target Slot Selection¶
A very important step when installing an update is to determine the correct mapping from the images that are contained in a RAUC bundle to the slots that are defined on the target system. The updated must also assure to select an inactive slot, and not accidentally a slot the system currently runs from.
For this mapping, RAUC allows to define different slot classes. A class describes always multiple redundant slots of the same type. This can be, for example, a class for root file system slots or a class for application slots.
Note that despite the fact that classic A+B redundancy is a common setup for many systems, RAUC conceptually allows any number of redundant slots per class.
Now, multiple slots of different classes can be grouped as a slot group. Such a group is the base for the slot selection algorithm of RAUC.
Consider, for example, a system with two redundant rootfs slots and two redundant application slots. Then you group them together to have a fixed set of a rootfs and application slot each that will be used together.
To detect the active slots, RAUC attempts to detect the currently booted slot. For this, it relies on explicit mapping information provided via kernel command line or attempts to find it out using mount information.
All slots of the group containing the active slot will be considered active, too.
2.5. Slot Status and Skipping Slot Updates¶
RAUC hashes each image or archive when packing it into a bundle and stores this hash in the bundle’s manifest file. This hash allows to reliably identify and distinguish the image’s content.
When installing an image to a writable file system, RAUC will write an additional slot status file after having completed the write operation successfully. This file contains the slots hash.
The next time RAUC attempts to install an image to this slot, it will first check the current hash of the slot by reading its status file, if possible. If this hash equals the hash of the image to write, RAUC will skip updating this slot as a performance optimization.
This is especially useful when having a setup with, for example, two redundant application file systems and two redundant root file systems. In case you update the application file system content much more frequently, RAUC will save update time by skipping the root file system automatically and only installing the changed application.
2.6. Boot Slot Selection¶
A system designed to run from redundant slots must always have a component that is responsible for selecting between the bootable slots. Usually, this will be some kind of bootloader, but it could also be an initramfs booting a special purpose Linux system.
Of course, as a normal user-space tool, RAUC cannot do the selection itself, but provides a well-defined interface and abstraction for interacting with different bootloaders (e.g. GRUB, Barebox, U-Boot) or boot selection methods.
In order to enable RAUC to switch the correct slot, its system configuration must specify the name of the respective slot from the bootloader’s perspective. You also have to set up an appropriate boot selection logic in the bootloader itself, either by scripting (as for GRUB, U-Boot) or by using dedicated boot selection infrastructure (such as bootchooser in Barebox).
The bootloader must also provide a set of variables the Linux userspace can modify in order to change boot order or priority.
Having this interface ready, RAUC will care for setting the boot logic appropriately. It will, for example, deactivate the slot to update before writing to it and reactivate it after having completed the installation successfully.
2.7. Installation and Storage Handling¶
As mentioned above, RAUC basically writes images to devices or partitions, but also allows installing file system content from (compressed) tar archives.
In addition to the need for different methods to write to storage (simple copy for block devices, nandwrite for NAND, ubiupdatevol for UBI volumes, …) the tar-based installation requires additional handling and preparation of storage.
Thus, the possible and required handling depends on both the type of input image (e.g. .tar.xz, .ext4, .img) as well as the type of storage. A tar can be installed on different file systems while an ext4 file system slot might be filled by both an .ext4 image or a tar archive.
To deal with all these possible combinations, RAUC provides an update handler algorithm that uses a matching table to define valid combinations of image and slot type while specifying the appropriate handling.
2.8. Boot Confirmation & Fallback¶
When designing a robust redundant system, update handling does not end with the successful installation of the update on the target slots! Having written your image data without any errors does not mean that the system you just installed will really boot. And even if it boots, there may be crashes or invalid behavior only revealed at runtime or possibly not before a number of days and reboots.
To allow the boot logic to detect if booting a slot succeeded or failed, it needs to receive some feedback from the booted system. For marking a boot as either successful or bad, RAUC provides the commands status mark-good and status mark-bad. These commands interact through the boot loader interface with the respective bootloader implementation to indicate a successful or failed boot.
As detecting an invalid boot is often not possible, i.e. because simply nothing boots or the booted system suddenly crashes, your system should use a hardware watchdog to during boot and have support in the bootloader to detect watchdog resets as failed boots.
Also you need to define what happens when a boot slot is detected to be unusable. For most cases it might be desired to either select one of the redundant slots as fallback or boot into a recovery system. This handling is up to your bootloader.