Here is the doc: https://www.drupal.org/documentation/modules/syslog
Basically, just enable syslog module which is already in the core. And disable the database logging module.
Enabled syslog module on Drupal’s module administration screen, went to Administration / Configuration / Development / Logging and errors and set the Syslog facility to LOG_LOCAL3.(Any of them in the dropdown should be ok I think)
Then you need config the syslog config file. It’s different between Linux and Mac. Here is a instruction for OS X Yosemite:
1. Just like above, create the log file with
sudo touch /var/log/drupal.log
2. Instead of adding a line to /etc/syslog.conf, add the following line to /etc/asl.conf:
# Drupal logging
? [= Facility local3] file /var/log/drupal.log
3. Restart logging with
sudo launchctl unload /System/Library/LaunchDaemons/com.apple.syslogd.plist
sudo launchctl load /System/Library/LaunchDaemons/com.apple.syslogd.plist
My problem is above restart syslog service not working. So I did a restart for my Mac and all works.