On Jan 18, 2008 9:53 PM, Steve Summit scs@eskimo.com wrote:
Do the load balancers in WMF's server farms attempt to associate sessions with servers? At first I thought this unlikely, but I guess it could significantly improve the hit rate.
Most likely you are seeing the effect of caching in the second tier squids. LVS distributes connections to the front end squids, front end squids do a tiny amount of memory only caching, then distribute requests to back end squids *based on the hash of the URL*.
This provides a tremendous gain in effective memory and disk storage (not storing hundreds of copies of every object), and the skinny cache on the front ends prevent single object demand spikes from taking out the site.
[snip; out of order]
When we were just having those stale-cache issues, with redirected pages and the like, it seemed that you got a consistently stale page, not the randomly stale page you'd expect
Also, on a frequently requested object if multiple caches get ahold of the object they'll all likely get purged or all not be purged, because the purging is triggered directly from mediawiki (via the spiffy multicast purger) and the caching lifetime is very long for objects which have not been purged.