If you apt-get install sysstat, and sar returns:
Cannot open /var/log/sysstat/sa27: No such file or directory
Sar needs to be enabled before it can be used. The error message in this case is completely useless, and the solution I have found is as below:
Enable sysstat data collection by doing
# dpkg-reconfigure sysstat
or manually by changing value of ENABLED from "false" to "true" in "/etc/default/sysstat".
Then start sysstat via:
# /etc/init.d/sysstat start
Check "/var/log/sysstat/" for the missing file.
Run sar after about 10 minutes to see the collected data.
# sar -A