Discovered while working on T205870: Fully migrate producers off statsd, looks like varnishreqstats sends truncated statsd traffic:
root@graphite1004:~# ngrep -q -W byline . udp dst port 8125 | grep -v -e '^U ' -e '^$' | grep -v \| interface: eno1 (10.64.16.0/255.255.252.0) filter: (ip or ip6) and ( udp dst port 8125 ) match: . varnish.ulsf varnish.uls varnish.ulsfo.upload.frontend.request varnish.eqiad.upload.frontend.requ varnish.codfw.upload.frontend.request.backend varnish.eqsin.upload.frontend.request.b varnish.eqsin.upload.frontend.request.bac varnish.esams.upload.frontend.req varnish.eqiad.upload.frontend.request varnish.esams.upload.frontend.re varnish.eqsin.upload.frontend.request.bac varnish.codfw.upload.frontend.request.client.s varnish.ulsfo.upload.frontend.request.clie varnish.esams.upload.frontend. varnish.e varnish.eqsin.upload.frontend.request.bac varni varnis varnish.codfw.text.fron varnish.esams.upload.frontend.re varnish.esams.upload.frontend.re varnish.esams.upload.frontend.req varnish.esams.upload.frontend.requ
From a cursory look at the code it seems to me CacheStatsSender doesn't take into account udp MTU to break up metrics and thus under high traffic partial udp packets are sent out. OTOH varnishstatsd doesn't seem affected as it does take into account MTU.