linux – Which files are kept during upgrades of Synology NAS OUTSIDE of user data like “/volume1”?

For historical reasons, some NAS of my customers have customizations in /etc/crontab to regularly execute things and in /etc/profile to provide customized login shells for SSH/PuTTY. Additionally, login with SSH/PuTTY supports using a key pair, so the file /root/.ssh/authorized_keys exists and is preserved during upgrades as well. While I’ve read that all contents of / get wiped out during updates, things don’t seem to be that easy or /etc/crontab and /root/.ssh/authorized_keys wouldn’t be available anymore.

So here’s what I’ve found so far: Synology uses upstart and that fires the job /usr/share/init/root-file-system.conf at some point, which itself uses a script named /usr/syno/sbin/config_bootup_check.sh. That really seems to delete /etc on every reboot and replace that with the contents of /etc.defaults. This is what makes customizations in /etc/profile disappear and can be worked around e.g. by placing those customizations into /etc.defaults/.bashrc_profile instead, which gets sourced by /etc/profile automatically. THOUGH, while .bashrc_profile survives reboots, it DOES NOT survive updates.

The only workaround I can think of is creating some trigger for the task planner, as that supports executing actions on reboot, which checks if /etc.defaults/.bashrc_profile is customized or not already and does so as necessary. That shouldn’t slowdown the boot too much and should be somewhat safe, as /etc.defaults/.bashrc_profile seems to be designed to only change during updates.

Though, that doesn’t explain how /etc/crontab and /root/.ssh/authorized_keys are preserved not only during reboots, but updates as well. Looking at /root/.ssh, it has a timestamp from applying the last update, while the files within have much older timestamps. Additionally I’m somewhat sure that /usr/syno/sbin/config_bootup_check.sh really only deletes directories like /etc, not /root. But that doesn’t explain how /etc/crontab is kept.

So, are there any docs available about which files are kept during reboots, updates etc.?