keeping previous kernel for recovery?

[belegdol]

Please forgive me if it is a stupid question, but what is preventing us from keeping n previous kernels installed in case an update is broken and does not boot? Fedora does it, Ubuntu does too. In general it should be simpler to boot a previous kernel than to try to pull the SD card and fix it in a second machine.

[SteeMan]

This design decision has been around from the early days of Armbian and SBC's.  If you think about Armbian as a firmware for your board instead of a general operating system like on your PC, that makes some sense as to why originally that would have made the most sense (you install new firmwares, you don't upgrade them in place).  Also, all those early boards were limited on the amount of internal emmc storage they may have had, if any, and SD cards were also much smaller in storage capacity at that time.  So wasting space with an entire extra kernel wasn't really an option.

 

Now move forward 10 years and most of those limitations are lessened.  So it could be supported today for most build targets.  But the current approach would likely still be needed for some boards.  So while it would be feasible to support having multiple kernels installed it would take a lot of effort to implement (across all the different booting scenarios, u-boot scripts, extlinux, uefi, etc).  Also in many of these scenarios it doesn't really help as you need an interactive boot (i.e. with hdmi and keyboard/mouse input) to present a menu to the user to select an alternate boot target, and that interactive boot environment isn't common on SBC's (unlike the PC world).  If you have to solder and hookup a USB-UART connector to monitor the debug output to be able to switch your boot kernel, is that better/easier then just popping in a different SD card?

 

So ultimately like a lot in the open source world, this change while possible, hasn't attracted the interest/need for any developer out there to want to undertake this effort.  Which leads me to the inevitable next comment, PRs are always welcome for contributions to the project 🙂

 

(Personally this is something I would also like to see, but in looking into the effort it would take, I know I personally don't have the time to implement, and the workarounds are good enough for me)