I'm working on a site with Varnish enabled, but I notice that the system clear cache pages, according to the expire configuration, also if pages are not changed. Is it possibile to clear cache only if a page is changed?
You can achieve this by setting a very high default TTL (expire time) and then invalidating the cache only for modified content. The Varnish module and the Cache Actions module are useful tools for that purpose.
Fabian Sörqist, maintainer of the Cache Actions module, has written a detailed article about it here: http://dev.nodeone.se/en/caching-with-varnish-drupal-7-and-cache-actions