Monitoring time sync with node_exporter

`ntp` collector

NOTE: This collector is deprecated and will be removed in the next major version release.

This collector is intended for usage with local NTP daemons including ntp.org, chrony, and OpenNTPD.

Note, some chrony packages have local stratum 10 configuration value making chrony a valid server when it is unsynchronised. This configuration makes one of the heuristics that derive node_ntp_sanity unreliable.

Note, OpenNTPD does not listen for SNTP queries by default. Add listen on 127.0.0.1 to the OpenNTPD configuration when using this collector with that package.

`node_ntp_stratum`

This metric shows the stratum of the local NTP daemon.

Stratum 16 means that clock are unsynchronised. See also aforementioned note about default local stratum in chrony.

`node_ntp_leap`

Raw leap flag value. 0 – OK, 1 – add leap second at UTC midnight, 2 – delete leap second at UTC midnight, 3 – unsynchronised.

OpenNTPD ignores leap seconds and never sets leap flag to 1 or 2.

`node_ntp_rtt`

RTT (round-trip time) from node_exporter collector to local NTPD. This value is used in sanity check as part of causality violation estimate.

`node_ntp_offset`

Clock offset between local time and NTPD time.

ntp.org always sets NTPD time to local clock instead of relaying remote NTP time, so this offset is irrelevant for this NTPD.

This value is used in sanity check as part of causality violation estimate.

`node_ntp_reference_timestamp_seconds`

Reference Time. This field show time when the last adjustment was made, but implementation details vary from "local wall-clock time" to "Reference Time field in incoming SNTP packet".

time() - node_ntp_reference_timestamp_seconds and node_time_seconds - node_ntp_reference_timestamp_seconds represent some estimate of "freshness" of synchronization.

`node_ntp_root_delay` and `node_ntp_root_dispersion`

These values are used to calculate synchronization distance that is limited by collector.ntp.max-distance.

ntp.org adds known local offset to announced root dispersion and linearly increases dispersion in case of NTP connectivity problems, OpenNTPD does not account dispersion at all and always reports 0.

`node_ntp_sanity`

Aggregate NTPD health including stratum, leap flag, sane freshness, root distance being less than collector.ntp.max-distance and causality violation being less than collector.ntp.local-offset-tolerance.

Causality violation is lower bound estimate of clock error done using SNTP, it's calculated as positive portion of abs(node_ntp_offset) - node_ntp_rtt / 2.

`timex` collector

This collector exports state of kernel time synchronization flag that should be maintained by time-keeping daemon and is eventually raised by Linux kernel if time-keeping daemon does not update it regularly.

Unfortunately some daemons do not handle this flag properly, e.g. chrony-1.30 from Debian/jessie clears STA_UNSYNC flag during daemon initialisation and does not indicate clock synchronization status using this flag. Modern chrony versions should work better. All chrony versions require rtcsync option to maintain this flag. OpenNTPD does not touch this flag at all till OpenNTPD-5.9p1.

On the other hand combination of sync_status and offset exported by timex module is the way to monitor if systemd-timesyncd does its job.

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