The node exporter provides kernel file descriptor metrics.
The kernel file descriptor metrics are pretty simple, there's only two of them which come from file-nr and file-max:
# HELP node_filefd_allocated File descriptor statistics: allocated. # TYPE node_filefd_allocated gauge node_filefd_allocated 17843 # HELP node_filefd_maximum File descriptor statistics: maximum. # TYPE node_filefd_maximum gauge node_filefd_maximum 3.273286e+06
They mean what you'd think they mean, the kernel has a limit for how many file descriptors everything in the system can simultaneously have allocated. This is based on how much memory your machine has by default, and running out would be bad. It is however extremely unlikely that you'd ever run in to this limit.
Where there is confusion is that the
node_filefd_* metrics are machine-level kernel metrics - as you'd expect from the node exporter. If you're worried about running out of file descriptors, what you almost certainly want are the per-process file descriptor metrics
I've seen this cause confusion for users, and not just of Prometheus. Unless you're running a system that's expected to e.g. be handling millions of concurrent network connections such that this limit is actually a potential concern, there's no need to have
file-nr from sar etc.) on a dashboard.
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