[Mageia-dev] Why ntpdate still there?

[Colin Guthrie]

'Twas brillig, and Pierre Jarillon at 11/02/13 11:19 did gyre and gimble:
> ntpdate is no longer necessary because ntpd includes all functions necessary 
> to set the clock.
> 
> Use of ntpdate in crontab is a very bad idea because a great number of  users 
> can make a burst of activity and a saturation of servers.
> ntpd is more accurate with few requests and can also setup the clock.
> ntpd makes requests after a long time. More the clock is adjusted, more it 
> waits before a new request.
> 
> 
> In man ntpdate, we can see:
>  Disclaimer:  The functionality of this program is now available in the ntpd 
> program. See the -q command line option in the ntpd - Network Time Protocol 
> (NTP) daemon page. After a  suitable period of mourning, the ntpdate program 
> is to be retired from this distribution.
> 
> ntpdate and ntp are active in drakxservices... IMO, this is a nonsense.

ntpdate is a service that is run prior to ntpd itself. This was done
previously to ensure the time was set correctly due to a (deliberate)
limitation in ntpd itself which did not change the clock when large
jumps were involved. It attempts to keep the change continuous and
avoids large jumps.

See this section from the man page:

       Under ordinary conditions, ntpd slews the clock so that the time
is effectively continuous and never runs backwards. If due to extreme
network congestion an error spike  exceeds  the
       step  threshold,  by  default 128 ms, the spike is discarded.
However, if the error persists for more than the stepout threshold, by
default 900 s, the system clock is stepped to the
       correct value. In practice the need for a step has is extremely
rare and almost always the result of a hardware failure. With the -x
option the step threshold is increased to 600  s.
       Other options are available using the tinker command on the
Miscellaneous Options page.

       The  issues  should  be carefully considered before using these
options. The maximum slew rate possible is limited to 500
parts-per-million (PPM) by the Unix kernel. As a result, the
       clock can take 2000 s for each second the clock is outside the
acceptable range. During this interval the clock will not be consistent
with any other network  clock  and  the  system
       cannot be used for distributed applications that require
correctly synchronized network time.


So ntpdate as a service is just a one-shot thing, it happens once at
boot to ensure the clocks are properly set and then ntpd takes over for
the rest of the time that machine stays up.

As far as I'm aware, there is nothing integrated into crontab regarding
ntpdate, but please feel free to correct me on that one.

Col



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Colin Guthrie
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http://colin.guthr.ie/

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