On Debian, Apache installed from repositories has mod_expires turned off by default. This seriously increases server load.
So what does mod_expires do? It "controls the setting of the Expires HTTP header and the max-age directive of the Cache-Control HTTP header"...
Let me explain.
The module allows apache to inform clients (eg. You browser) about the length of time a particular asset can treat as valid.
Drupal 7 in its .htaccess has these lines
# Enable expirations.
# Cache all files for 2 weeks after access (A).
This piece of code (if you have mod_expires enabled), informs all clients that files which are not *.php files are valid for 2 weeks.
If your browser fetches an image from a server once, it will hold it in its cache and serve it from memory for 2 weeks, instead of fetching it from the server every time you load a page.
The biggest gain is usually achieved on images. All images for your website's template and all content images are requested only once every 2 weeks. On a website with 10 images, one user who visits the page every 3 days, makes a total of 10 requests instead of 50 during these 2 weeks. Assuming he visits 3 pages every single visit, you might save yourself 140 requests to Apache. 14 times less requests!!!
Without mod_expires Varnish underperforms
If you did not enable mod_expires, the varnish is seriously underperforming on your server.
In performance config on your Drupal website (admin/config/development/performance), you can set cache expiration for cached pages. This, however, works only for the page (HTML and for the css/js files).
Requests for images are separate requests and if mod_expire is disabled max-age directive nor EXPIRES heard are set on these files. Varnish treats them as files which are valid only for a single request and does not cache them. Each request is forwarded to Apache.
Back to our example, if we have a website with 10 graphical resources, which stands behind varnish, if you have mod_expires disabled, w visiting user will do:
- min 2 requests to varnish (page and CSS file) - headers set by Drupal
- 10 requests to fetch images
If you turn on mod_expires, all 12 requests can be served by varnish, not even touching your hard drives.
Thanks to mod_expires, your varnish hit rate will improve significantly and your hard drives will last much longer.
So do you have mod_expires enabled?
If not then:
sudo a2enmod mod_expires sudo service apache2 reload