Proxy buffering query

Francis Daly francis at
Wed Aug 3 23:55:34 UTC 2022

On Wed, Aug 03, 2022 at 08:49:20AM -0400, libresco_27 wrote:

Hi there,

> But I'm not explicitly defining the value for proxy_busy_buffers_size to
> something. Right now it is set with the default value

Oh, sorry. I had misunderstood what you were reporting.

I now think that the confusion comes from what "the default value"
for proxy_busy_buffers_size is.

The documentation has one set of words, which is correct once you know
how it is intended to be interpreted.

If we can find a set of words that is both correct and clear, we can
probably get the documentation changed to help the next person.

I think that the key is: if not explicitly defined, the value for
proxy_busy_buffers_size is "the bigger of: twice proxy_buffer_size;
and the size of two proxy_buffers".

By default-default, that is the 8k or 16k that the documentation summary

But when you set a big proxy_buffer_size value, you are implicitly
increasing proxy_busy_buffers_size as well.

And then the other requirement kicks in -- proxy_busy_buffers_size must
be not bigger than "proxy_buffers number-minus-one times size".

Also, if you want to explicitly set proxy_busy_buffers_size, you cannot
make it be smaller than a single proxy_buffers, or than proxy_buffer_size.

So with all default values on a 4k page system, proxy_buffers is "8
4k" (32k total, and 28k is the maximum for proxy_busy_buffers_size);
proxy_buffer_size is 4k; and proxy_busy_buffers_size is 8k.

If you want to set proxy_buffer_size to more than 14k, you must either
also increase proxy_buffers (in number or size); or explicitly set
proxy_busy_buffers_size to 28k or lower (while not being smaller than

Hopefully this does not make things more confusing...


Francis Daly        francis at

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