[ntp:questions] Query about NTP accuracy

Richard B. Gilbert rgilbert88 at comcast.net
Mon May 25 02:04:59 UTC 2009


Garrett Wollman wrote:
> In article <ywn9zld20zs5.fsf at ntp1.isc.org>,
> Harlan Stenn  <stenn at ntp.org> wrote:
> 
>> I agree that ntpd -g is usually better than sntp (or ntpdate) to initially
>> set the time, but the choice is between "Set the clock well, even if it
>> takes a little time" and "Set the clock as quickly as possible and it may be
>> wrong."
> 
> What is actually needed at boot time is usually neither of these
> things.  It's more like "if the clock needs to be stepped backwards,
> figure this out as quickly as possible, before any important services
> start, accepting that there is some small chance that an additional
> step in either direction may be required."  It's a straightforward
> trade-off of the sort engineers make all the time: there's a limit to
> how much one is willing to pay (in initiailization time) to reduce the
> likelihood of post-startup nonmonotonicity.  This limit varies
> depending on the application.
> 

The pain may be avoided by simply not restarting servers.  This is not 
possible for some shops but for those that can or must run 24x7 it's the 
easiest and best solution.  NTPD takes a while to pull into tight 
synchronization; in some cases as long as ten hours, so there's a big 
payoff!




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