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user630702
Asked: 2020-05-02 05:12:16 +0800 CST

NTP - After how much delay would NTP select a remote server in the pool?

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A selected server will be indicated by *. But how much time will this take to select a peer from the pool if ntp service is restarted?

I'm trying to monitoring NTP server offset but every time i restart the service the selection process takes a while and wanted to know how long it'd take.

~$ ntpq -p
remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*time4.stupi.se  .PPS.            1 u    2  128  377    7.796   16.636   4.146
+213-21-116-142. 192.36.144.22    2 u   18   64  377    8.261   17.013   3.807
+ntp.xpd.se      192.36.144.22    2 u   38   64  377    8.855   20.929   2.537
+mail.joacimmeli 192.36.144.22    2 u   28   64  377    9.228   19.509  25.804
+juniperberry.ca 193.79.237.14    2 u   26   64  377   44.612   19.835   2.443
~$
ntp ntpd

1 Answers

  1. Best Answer

    According to the NTP docs, it takes a minimum of 6 polls to achieve synchronization. With pools, the recommended way to achieve this is by use of the iburst option on the pool directive, e.g.

    pool pool.ntp.org iburst

    By using iburst, you should achieve synchronization with the first contact of the pool hosts. This should take no more than a few seconds (depending on your upstream bandwidth), so 10 minutes seems a reasonable cutoff for lack of synchronization.

    Normally, ntpd will select a few hosts from each pool defined in the configuration, then discard the less accurate peers over time, settling on the best hosts after a few polls - usually it's 4 hosts, but that's just my observation and isn't documented behaviour, as far as I know.

    If you have operational requirements for NTP monitoring, you might want to consider using an existing monitoring solution rather than a home-grown one. I wrote an overview of NTP alerting & telemetry solutions a few years ago which might be helpful.

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