sar: enable data collecting

I've just installed the sysstat package on Manjaro. When running sar for the first time I see:

Cannot open /var/log/sa/sa21: No such file or directory
Please check if data collecting is enabled

How do I check whether data collecting is enabled?

If it's not, how would I go about enabling it?

On Ubuntu 16.04 I just ran into this same issue. After some grousing around I found I needed to enable the service by editing the file /etc/default/sysstat

Change ENABLED="false" to true:

# Should sadc collect system activity informations? Valid values
# are "true" and "false". Please do not put other values, they
# will be overwritten by debconf!
ENABLED="true"

Restart the service:

systemctl restart sysstat.service

Just a quick note, like a dingleberry I wasn't reading closely enough and navigated to /etc/sysstat/sysstat instead of /etc/default/sysstat and wasted 5 minutes poking around the man page and stuff.

The sysstat service and associated timers need to be started:

systemctl start  sysstat sysstat-collect.timer sysstat-summary.timer

To auto-start on next system startup:

systemctl enable sysstat sysstat-collect.timer sysstat-summary.timer

Normally, this would be run by cron or as a service depending on how the package for your system configures it. Modern versions use sa1 to collect the historical data to files. Older versions ran atsar to do the same.

Check the man pages starting with man sar. Also there should be documentation in /usr/share/doc/sysstat.

sar can be used to collect and display current data even if the historical data is not available.

Check to see if there are any files in /var/log/sa, If there are then data is being collected.