Deleting syslog's logfile without doing a reload of syslog daemon is no good idea because the file is still opened by the daemon. You can either rename the file and then force a reload of syslog or you stop syslog, delete the file and start it again. Another way of preventing root partition from getting filled would be to put logfile to another partition. you can edit /etc/syslog.conf and change path to logfiles. But you should not put it to MyDocs as this may prevent partition from being unmounted and used via usb.