I have a suggestion. Maybe you need to use rotation logs, because when many people visit a sites, the log file has a very large size.
My rotation configuration in "/etc/logrotate.d/cwp"
/usr/local/apache/logs/*_log {
missingok
notifempty
sharedscripts
delaycompress
postrotate
/bin/kill -HUP `cat /usr/local/apache/logs/httpd.pid 2>/dev/null` 2> /dev/null || true
endscript
}