Nginx for a site allowing downloads

My friend have a site that allows users to download files.However the moment one guy start downloading server load change from 0.00 to 3.50 in a matter of minutes.Nginx and php-fpm reach D state and IOwait is like 100% on all cpu cores.I tried to limit number of concurrent connection but this seems to have no effect on server load.

user              nginx;
worker_processes  4;



events {
    worker_connections  1024;
}


sendfile        on;
#tcp_nopush     on;
gzip  on;
#keepalive_timeout  0;
keepalive_timeout  2;

My php-fpm setting

pm.max_children = 7
pm.start_servers = 5
pm.min_spare_servers = 5
pm.max_spare_servers = 6
pm.max_requests = 500

Server - 8gb ram Quad core processor 1TB sata hdd