Load balance -- Write IO is too high

Hi guys:

i have two servers, and 1 server as proxy server and upstream 1 server
and 2 server.

I set up 1 server’s weight is 1, and 2 server’s weight is 5 .

Now they can work properly , but 1 server’s write IO is too hight .
Applications is no write operation , Why such a high level of write IO ?

Thanks very mach. Sorry for my bad english