I've read the sadc man page and found that if I want to track disk usage stats I need to execute sadc with the -d flag. So I modified the /etc/init.d/sysstat script to start up sadc with the -d option. This appears to work as expected to start with. However after the daily roll of the stats files in /var/log/sa the -d value seems to get forgotten as the next day's file does not have disk stats. I have been looking for what is restarting the file daily but I'm not seeing an obvious answer there to see if I just need to add -d somewhere else. Tracking disk stats have value to us as we are using a number of SAN attached storage arrays on a server and we wish to track which of these devices are being used and how they are being used.
I'm not sure whether you have not edited /etc/cron.d/sysstat file but there is necessary to edit /usr/lib{64}/sa/sa1 script - which creates an output record to /var/log/sa/sa{date} file per 10 minutes (see /etc/cron.d/sysstat). /etc/init.d/sysstat only creates the initial record to /var/log/sa/sa{date}.