If your intention is to totally remove the space that's occupied by the log file, you'll probably want to pipe /dev/null into the file:
cat /dev/null > /path/to/logfile
cat /dev/null > /path/to/logfile
Your command will make /path/to/logfile fill up all available space on the disk. To truncate that file, which is probably what you wanted to do, you should
`echo > /path/to/logfile`