[systemd-devel] What's the best way to generate .conf files on startup?

[Lennart Poettering]

On Mo, 22.02.21 23:22, Sean McKay (insanescientist at gmail.com) wrote:

> Hi all,
>
> I've been looking to dynamically create .conf files at boot depending on
> the hardware that I'm running on (to set MemoryMax, if it's relevant). I'd
> assumed that the proper way to do this would be by using a generator, since
> the .conf file won't automatically be loaded and would require triggering
> the system manager to reload all configuration. And it didn't seem prudent
> to call that *during* boot.
>
> Then I ran across this little snippet in the man page for
> systemd.generator, which seems to imply the opposite:
> Generators should only be used to generate unit files and symlinks to them,
> not any other kind of configuration. Due to the lifecycle logic mentioned
> above, generators are not a good fit to generate dynamic configuration for
> other services. If you need to generate dynamic configuration for other
> services, do so in normal services you order before the service in question.
>
> What I'm not clear on is whether this refers solely to configuration used
> by the daemon itself (to use sshd as a well known example - eg:
> /etc/ssh/sshd_config) or if it also refers to drop in .conf files (ie:
> something in /run/systemd/system/ssh.service.d/)
> Put differently, is a drop in considered to be a unit file or to be
> configuration (for the purposes of the above helptext)?

It's considered a unit file. I figure we should improve the docs on
that, to make this clearer.

> Would the recommended solution in this case be for me to use a generator to
> create the relevant .conf file(s) for MemoryMax? Or would it be better to
> use a normal service (with proper ordering against the ones it's modifying)
> to generate those .conf files and call daemon-reload during boot? If the
> latter, are there any expected risks associated with calling daemon-reload
> during boot?

Doing this with a generator sounds perfect to me.

Lennart

--
Lennart Poettering, Berlin