I have a large Wordpress site (600-900 consistent users at all times) and about 6 months ago we setup Varnish. Our config uses 5GB cache and is stored in memory. When the site hits around 1500-2000 people at once then Varnish starts to throw 503 errors. We used to store the cache on disk, but moving this off to memory has seemingly reduced the 503 errors.
We have 3 iFrames on our site that we use on each page for inline ad refresh. I noticed in varnishlog that each time a user hits a page, it counts as 4 hits (one to the actual page + 3 iframes containing the ad codes). That means 1500 users on the site actually equates to 6000 varnish hits (if none of them miss). Does this matter, as there is basically nothing on the iFrame pages themselves except ad codes? Or can the sheer amount of hits to varnish cause performance problems?
If it can cause Varnish to crash from too many requests, I might consider telling it to stop caching the iFrame URLs, but then I am worried about all of the hits I will take to Apache...
Thanks!
