[systemd-devel] Reducing unmount/mount of partitions on soft-reboot

[Luca Boccassi]

On Wed, 13 Mar 2024 at 12:30, Aditya Gupta <adityag at linux.ibm.com> wrote:
>
> Hello,
>
> I tried systemd-soft-reboot on a RHEL system, and it's amazing in terms
> of it's ability to do a userspace reboot, within fraction of time of a
> full system reboot. For example, for a Power system taking around 50
> seconds to do a normal reboot, it took around 4-5 seconds for a
> systemd-soft-reboot.
>
> I have a question on further optimisation. After soft-reboot, I notice
> much of the time is taken up by .device and .mount services. This was my
> observation based on 'systemd-analyze blame'. Please do let me know if
> I am seeing the wrong numbers, or if there's a better way to know.
>
> Is there some way to 'pass-through' these mounts ? That is, I might not
> need to unmount and remount my boot/root paritions.
>
> I tried finding this in available documentation, sorry if I missed it. I
> did find this ability to 'pass-through' resources/file
> descriptors/sockets to next boot, by modifying the service files, but
> don't know how and if I can do a similar thing with mounts.

Pre-prepare the additional mounts in /run/nextroot/, after preparing
the new rootfs, so that they are ready ahead of time.