alt Hacker News6.19 added a new Live Update Orchestrator, which allows significantly more of the system to be retained while doing a kexec / Kernel Handover like transisiton to a new kernel too. https://www.phoronix.com/news/Linux-6.19-Live-Update-LUO https://lwn.net/Articles/1033364/
Systemd added support in recent 2.61. Theres also now ways to have user stores, that survive across switches. https://www.phoronix.com/news/systemd-261
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I'm glad to see this. Almost 18 years ago I implemented a similar kexec device+memory preservation for a storage vendor. It was done on a Linux kernel of that day, and it had had a memory reservation and handoff protocol between the two kernels to keep some specific PCI device alive, allowing for state restoration at the application side. I'm proud of the fact that the kernel replacement was just under 1 second in execution (after init process optimization) and the whole kernel+app was less than 10 seconds.
There's also `systemctl soft-reboot` which initiates a userspace-only reboot, which quickly restarts the system without going through the full hardware and kernel initialization process.