Some customers of an internal Kubernetes platform complain that their pods keep getting evicted because their nodes keep running out of disk space. The platform's maintainers' first instinct was that customers' pods should not write to ephemeral storage, e.g., should not write to log files on the filesystem unless mounted from external storage. But that turns out not to be the case: customers' pods did not write anything to disk at all. So why were nodes running out of disk space? Prometheus metrics show which partitions use how much disk space but cannot go into more detail. The team wanted to inspect the node's filesystem to figure out what exactly is using so much disk space. The first thing they tried is to run a management pod that contains standard tools such as 'df' and that mounts the host's filesystem. Unfortunately the act of scheduling a pod on that node causes it to experience disk pressure, and so the management pod gets evicted.

So, being dogmatic about "the host should not have any tools installed" is good and all, but how do you debug this scenario without tools on the host?

We eventually figured it out. By logging into the host OS and using the shell tools there.