I recently upgraded my wikis from 1.24.2 to 1.26.2 and I'm trying to
diagnose why my Varnish hit ratios (4 web servers) dropped from ~85% to
~37%. Nothing in my Varnish configuration or VCL changed. For what it's
worth, I allocate 4 GB to Varnish. I'm fairly sure it has to do with
image/thumbnail caching, since there's about 30 GB worth of thumbnails
across all five wikis, though 12 GB of that is for our biggest and most
heavily trafficked wiki. (FWIW, I did change vcl_recv() last year to strip
cookies from thumbnail images and that improved overall performance while
maintaining a high hit ratio.) The Varnish n_lru_nuked stat is growing
rapidly as well, so it's definitely using all of the memory and having to
make room for new stuff.
Any thoughts on what might be causing this degradation, or at least how to
diagnose it?