nginx log de erros: r ocupado novamente durante o envio de resposta ao cliente,

Question

Recebi muitos erros no meu log de erros do nginx da seguinte forma:

2015/04/28 20:05:32 [alert] 67449#0: *29938935 sendfile(/var/www/images/201504/13/210123jpt6cpjj6r1g2upt.jpg) returned busy again while sending response to client, client: 113.12.147.225, server: www.example.com, request: "GET /data/attachment/forum/201504/13/210123jpt6cpjj6r1g2upt.jpg HTTP/1.1", host: "www.example.com", referrer: "http://www.example.com/thread-1273026-1-4.html"
2015/04/28 20:05:32 [alert] 67442#0: *29938913 sendfile(/var/www/images/201504/13/210131npa0btbth44rl0lg.jpg) returned busy again while sending response to client, client: 113.12.147.225, server: www.example.com, request: "GET /data/attachment/forum/201504/13/210131npa0btbth44rl0lg.jpg HTTP/1.1", host: "www.example.com", referrer: "http://www.example.com/thread-1273026-1-4.html"
2015/04/28 20:05:32 [alert] 67443#0: *29938775 sendfile(/var/www/images/201504/13/210126in2xyyxuc78l4csy.jpg) returned busy again while sending response to client, client: 113.12.147.225, server: www.example.com, request: "GET /data/attachment/forum/201504/13/210126in2xyyxuc78l4csy.jpg HTTP/1.1", host: "www.example.com", referrer: "http://www.example.com/thread-1273026-1-4.html"
2015/04/28 20:05:32 [alert] 67452#0: *29938925 sendfile(/var/www/images/201504/13/210149mal5jh0jjh6jlxhj.jpg) returned busy again while sending response to client, client: 113.12.147.225, server: www.example.com, request: "GET /data/attachment/forum/201504/13/210149mal5jh0jjh6jlxhj.jpg HTTP/1.1", host: "www.example.com", referrer: "http://www.example.com/thread-1273026-1-4.html"
2015/04/28 20:05:32 [alert] 67455#0: *29935066 sendfile(/var/www/images/201407/16/204723k23k9bf900dfc3lc.jpg) returned busy again while sending response to client, client: 59.45.81.210, server: www.example.com, request: "GET /data/attachment/portal/201407/16/204723k23k9bf900dfc3lc.jpg HTTP/1.1", host: "www.example.com", referrer: "http://www.example.com/article-11248-1.html"

Já habilitei sendfile no nginx:

aio         on;
sendfile        on;
tcp_nopush     on;
tcp_nodelay on;
directio                    4m;
directio_alignment          4096;

Alguém sabe como corrigir esse problema?