Thanks for your reply!
Just to be sure:
You are in the file /usr/local/ispconfig/server/cron_daily.php?
Because that file has only 954 in my case.
And these lines of codes I can find according the tar backup:
//* Create a tar.gz backup as root user
$web_backup_file = 'web'.$web_id.'_'.date('Y-m-d_H-i').'.tar.gz';
exec('tar pczf '.escapeshellarg($web_backup_dir.'/'.$web_backup_file).' --exclude=backup\* --directory '.escapeshellarg($web_path).' .');
So the code
Should be changed with:
exec(ionice -c2 tar pczf