shared_buffers formula

Hello all!

Recently we've been having problems with swap on our postgresql server. It
has 125GB of RAM. We've decided to calculate it's memory consumption. To do
this we've used the formulas from the official docs [1]. However there is
one parameter that seems strange - Shared disk buffers. According to the
formula it occupies the following space:

(block_size + 208) * shared_buffers

Our values are

block_size=8192
shared_buffers=30GB

The block_size has the default value and shared_buffers was calculated by
pgtune. According to the formula the product will be around 252 000 GB
which doesn't make any sense. Is there something wrong with the formula? Or
should we make our shared_buffers approx. 10 000 times lower? :)

Thanks in advance,
Alexander Shutyaev

[1]
http://www.postgresql.org/docs/9.1/static/kernel-resources.html#SHARED-MEMORY-PARAMETERS