Re: Those damned reboots
"I don’t know how other mail servers (Linux, windows or other hosted) have handled updates and thus whether they are actually any better in this respect."
Looking back at emails from Mythic Beasts since May 2015 there have been just 5 which refer to the server which provides my email access being out of service. Two were for hardware upgrades and two for OS (Debian) major version upgrades. The other was unspecified. For the OS upgrades they were setting aside an window of 2 hours with the expectation of it only taking 30 minutes. On the basis that their servers are a bit niftier than my laptop and having done the equivalent Devuan upgrades I've no reason to disbelieve they achieved that.
What normally happens with less drastic Linux upgrades, say a new version of LibreOffice, is that new versions of programs or libraries are put in place and linked to the file names so that on the next invocation the new version is picked up and, when the old ones become redundant their storage allocation is released to the free list. To ensure that happens with a service such as the exim mail server the service is stopped and immediately restarted which happens PDQ. I've only once, in many years, seen a service which was so low level that it needed a reboot. Sometimes an update obsoletes a library entirely in which case the old one may be left there to be removed by the administrator or autoremoved. On a multi-user system the onlly person noticing what's happening will be the admin.
It's left to the new software to update per-user config files if that's needed. System-wide config files are also replaced but the admin might be given a choice as to whether to compare and leave the old versions in place. Updates to time zone data simply put in place the new file and restart the service.
With kernel updates the necessary files are files are put in place and the system can be rebooted as convenient to simply start running the new version (the service reboot I just mentioned was also on an as convenient basis). A reboot to a new kernel takes no longer than a reboot to an existing one - it's exactly the same process. How long it takes depends largely on the speed of the H/W and how many drivers, etc. have to be reinitialised. On my laptop, which is far from top flight because good enough is good enough, it takes about 42 seconds from pressing return on OS selection to having the desktop running, including time to enter the password. The process inevitably leaves the old kernel sitting alongside the new one and selectable at start time; some distros will remove the previous old version during the update, some will leave them for the admin to remove.
These days, at least for Debian, procedure for a complete version upgrade is not dissimilar except that everything gets replaced. The relevant config lines are pointed at the new version name and a variation of the normal update commands are run. The old system keeps running whilst replacing all its components underneath itself and then rebooted. (I think Mythic will have downloaded all the new files in order to get under 30 minutes.)
Basically, that's it. It Just Works, even at major OS upgrade level, let alone just updating a mail server.