On my server there are many virtual hosts and databases. I have a backup sh cron to create backup of the files every day at 4AM. From 4AM to 5AM, while backup is made, Apache fully becomes unreachable, it only gives timeout and 500 error. I'm using gzip to backup each virtual host. Gzip is eating only 50-100% of the CPU (the process in top, having a quad core CPU).