An excerpt from smartd manpage reads:
-l FACILITY, --logfacility=FACILITY
Uses syslog facility FACILITY to log the messages from smartd. Here FACILITY is one of local0, local1, ..., local7, or daemon [default]. If this command-line option is not used, then by default messages from smartd are logged to the facility daemon.
Followed by steps how to accomplish this on a syslog based system.
Apparently, if Arch Linux is up-to-date, it runs journald in conjunction with systemd for the same purpose. For the latter, I cannot find a way to configure facilities or whatnot.
How to give smartd a logfile of it's own in a journald environment?
I've also posted this on Unix & Linux Stack Exchange.
I too do not know. I want the smartd to log to /var/log/something but cannot figureout how.
The "facility" isn't what you think it is. It's not used to split into logfiles. It's just some arbitrary categorization. If you want all messages from smartd, run
$ journalctl -u smartd.service
If you want those message in a separate plain-text file, you need to run a syslog daemon.