Use uint64 in the ZFS collector

[fpletz]

ZFS metrics can also be unsigned 64-bit integers that won't fit in int64 and causes the whole collector to fail.

Example

time="2017-10-25T17:28:16Z" level=error msg="ERROR: zfs collector failed after 0.000335s: could not parse expected integer value for \"kstat.zfs.misc.zil.zil_itx_needcopy_bytes\"" source="collector.go:123"

# grep zil_itx_needcopy_bytes /proc/spl/kstat/zfs/zil 
zil_itx_needcopy_bytes          4    18446744073709037523

The internal ZFS datatype kstat_named_t can have an uint64 value: https://github.com/zfsonlinux/zfs/blob/master/lib/libspl/include/sys/kstat.h#L472

Why float64 and not uint64?

Prometheus uses float64 internally and the metrics would be converted to float64 eventually anyway.

[joehandzik]

This looks good to me, thanks for the catch. We saw cases like this in our Lustre Exporter, but hadn't in the ZFS code.

[fpletz]

The travis error is unrelated. Travis was apparently rate-limited by go.googlesource.com.

[SuperQ]

Rather than changing to float64, we could parse the proc file with uint64. We have done this in several other parts of the node_exporter/procfs library.

[joehandzik]

@SuperQ Any particular reason you'd advise avoiding the use of floats by default? We've erred on the side of just creating floats by default to prevent this sort of problem in other Go code. I haven't done any performance analysis comparing the two, curious if you have particularly strong reasons for what you suggested.

[SuperQ]

Mainly, I am aiming for correctness of parsing the files exposed by the kernel. ParseFloat() on a proc file could, in theory, give us strange results. This zfs parsing code should eventually be moved to procfs, where we would parse with uint64 since it would be a not-Prometheus-specific library.

It's not something that will happen any time soon, but there are vague ideas that we may support uint64 natively in the future at the metrics library level. For better compatibility with other metrics systems that do support it.

[SuperQ]

An example of how we want to go is the xfs parser.

[joehandzik]

@SuperQ Excellent clarification, thank you! @fpletz, sounds like it'd be for the best to just go with uint64 in this PR. I'll take a look at pulling the ZFS metrics into the procfs project.

[fpletz]

Sounds reasonable, thanks for the explanation! I'll change this PR accordingly.

[SuperQ]

If you want some real fun, you could port this code over to the procfs library. 😜

[discordianfish]

@fpletz As @SuperQ said, happy to merge it with uin64 but even better if you would move it to https://github.com/prometheus/procfs

[discordianfish]

@fpletz Can you change it to uint64 so we can merge it.
Otherwise I'll close this PR for now.

[fpletz]

@discordianfish Thanks for the reminder. Rebased and changed to uint64. Tests are green. Ready to merge.

[SuperQ]

LGTM

[discordianfish]

Great, thanks! LGTM!