Affect of copying /var/db/mysql folder under very heavy load -

i have backup script :

cd /var/db/mysql && tar -cvzf $backup/mysql-backup-sql.$now-$(date +"%h-%m-%s").tar.gz $dbs 

that shell script copies mysql folder , upload specified ftp adress.

but wonder:

my mysql under heavy load (daily +3,5 millions queries). if use script make backups, mysql server performance affected or there problem when cronjob copies mysql folder? safe way make hourly backups ? ..

thanks.