I'm trying to make an accurate estimate of CDN usage on Google Cloud Platform but am not sure about the fill costs.
Fill costs are incurred on a cache miss and the data is gotten from origin or another cache. What's not specifically mentioned is how granular a "cache" miss is. That is - is it a cache miss for the region? zone? POP? node?
With an international distribution this could be make a huge difference in estimation.
I asked Google support directly on this one and got back that cache fills occur in each "cache site." Or as they put it:
Cache fill is counted for each caching sites since cache fill occured between one cache location to another cache location.
The updated list of cache sites/locations is in their documentation.
At the time of writing that means a hypothetical max of 81 cache fills for a given result (not including expiring or being pushed out of the cache and re-filling etc.) - presuming your content is requested from each of these location as the cache is only filled when requested.