Setting large proxy_buffer_size

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Ran into an issue that I needed to set a larger proxy_buffer_size (e.g. to
128k). It works after increasing.

However my question is: what's the disadvantages of setting a large buffer
size? If there is no disadvantage, why the default is only 8k? Is there a
certain value that I certainly shouldn't set it larger than that?

Thanks!

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,233654,233654#msg-233654