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2018-07-02 The rabbithole that is /proc/self/mountstats [linux kernel prometheus golang go] Prometheus NFS client monitoring for UDP false

Everything started with a simple idea: Get per-client NFS statistics into Prometheus.

I found out that the Node Exporter is already able to export NFS client metrics via the mountstats collector. It gathers metrics from /proc/self/mountstats and exports them nicely for prometheus to scrape. Nice! Let's try this out:

./node_exporter --collector.mountstats
failed to parse mountstats: invalid NFS transport stats 1.1 statement: [740 1 881477 875055 5946 2888414103 286261 16 258752 2080886]

Hmm, that does not look good. After a lot of digging throught the code, I found out that the collector seems not to support NFS mounts via UDP! This is due to a different format of the mountstats file depending on the protocol. Here is an awesome writeup about the mountstats file format, which is seemingly now documented anywhere else. A big thanks to Chris Siebenmann! m,i The problem lies in the xprt line in /proc/self/mountstats, which contains transport statistics and looks like this for TCP:

xprt: tcp 695 1 1 0 16 96099368 96091328 6383 341933213458 1504192

All fields are explained in the link above. The crux is the following part:

For the udp RPC transport there is no connection count, connect idle time, or idle time (fields #3, #4, and #5); all other fields are the same.

This means that for UDP, the line contains three fewer fields than for TCP. The mountstats exporter always expects the same number of fields and therefore breaks for UDP.

Another tricky thing is the statvers variable in /proc/self/mountpoints that specifies which version the statistics refer to. statvers=1.1 added three more fields to the end, 11, 12 and 13 in the link above. I was not sure how this was handled for UDP, but after digging through kernel code and git logs, I found this commit, which shows that the three fields were added for both UDP and TCP.

I coded up the changes (learning some golang in the process) and opened a pull request with the procfs project in Prometheus at https://github.com/prometheus/procfs/pull/100, so the component now exports the statistics correctly for TCP and UDP. The fields that are missing in UDP are simply set to zero to ensure the same number of fields for both protocols. I also added a field specifiying the protocol, either "tcp" or "udp".