Loss of cache when persistent connexion is closed

Hello,
I have a problem with my install of postgresql. I have a program that
requests DB by opening persistent connexions. When the program is launched,
the disk IO are very high until postgresql cache is good enough (a few
hours). The problem is that when I stop the program all the connexions are
closed and so are the postrgesql processes. And the cache is lost.
I really want to use postgresql but today I use mssqlserver and it never
lost its cache till I stop it (mssqlserver). So I don't have problem with
disk IO when the program is stopped an restarted.

I assume that each postgresql process manage its own cache but that there is
no global cache. Am I right ? If true is there any possibility to have a
global cache for all processes and is it possible not to free this cache
when connexions are closed ?

Regards

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Guillaume Pungeot - mappy