In R77.x (or older) you can purge the active log file ($FWDIR/log/fw.log) in the SmartView Tracker GUI client.
In R80 you can make it from command line:
- Run: fw logswitch
This command will turn the active log file to a log file named after the date and time, and will open a new active log file - $FWDIR/log/fw.log. - Delete the the log file created by the previous command from $FWDIR/log directory on the Log Server. Delete all its pointer files as well - all files with the same prefix and different suffixes (.logaccount_ptr, .loginitial_ptr, .logptr).