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Mastering Embedded Linux Programming - Second Edition
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In this section, I will describe three approaches to applying software updates: symmetric, or A/B, image update; asymmetric image update, also known as recovery mode update; and finally, atomic file update.
In this scheme, there are two copies of the operating system, each comprising the Linux kernel, root filesystem, and system applications. They are labelled as A and B in the following diagram:

The bootloader has a flag that indicates which it should load. Initially, the flag is set to A, so the bootloader, loads OS image A. To install an update, the updater application, which is part of the operating system, overwrites OS image B. When complete, it changes the Boot flag to B and reboots. Now the bootloader will load the new operating system. When a further update is installed, the updater overwrites image A and changes the Boot flag to A, and so you ping-pong between the two copies. If an update fails before the Boot flag is changed, the bootloader...