Sizing your cache

Deciding on cache size can be a tricky task. A few things to consider:

• How big is your hot data set. For a portal or news site that would be the size of the front page with all the stuff on it, and the size of all the pages and objects linked from the first page.
• How expensive is it to generate an object? Sometimes it makes sense to only cache images a little while or not to cache them at all if they are cheap to serve from the backend and you have a limited amount of memory.
• Watch the n_lru_nuked counter with varnishstat or some other tool. If you have a lot of LRU activity then your cache is evicting objects due to space constraints and you should consider increasing the size of the cache.

Be aware that every object that is stored also carries overhead that is kept outside the actually storage area. So, even if you specify -s malloc,16G Varnish might actually use double that. Varnish has a overhead of about 1KB per object. So, if you have lots of small objects in your cache the overhead might be significant.