[Info-vax] Clock running very slow on an Alpha

[Richard B. Gilbert]

Mark Berryman wrote:
> On 8/25/10 2:12 PM, Richard B. Gilbert wrote:
> .
> .
> .
>>
>> The maximum error that a standard NTP can correct is 43 seconds per day!
>>
> 
> That is not at all correct.  NTP is capable of correcting any amount of 
> error.  You simply set parameters that define how much error is fixed by 
> slewing the clock vs when the clock is corrected in a single step.
> 
Alright!  43 seconds per day is the largest error correctable without 
stepping the clock or tampering with the parameters used by NTPD.
43 seconds per day represents a slew rate of 500 PPM, the largest slew 
rate that the stock NTPD will use.

> I have had systems come up with a date several years wrong because the 
> TOY clock does not keep year information and a backup had just been 
> restored.  NTP updated the clock just fine in these cases.
>
Startup is a special case.  You can tell NTPD to figure out what time it 
is and set that time.  Once the system is running, stepping the clock 
and especially stepping it backward can wreak havoc with programs that 
expect that time will be monotonically increasing.

> NTP routinely corrects errors of 3600 or more seconds twice a year for 
> many systems.  That is why the sanity timer usually defaults to a value 
> of 4000.

If you are thinking of the switch from daylight savings to standard time 
and vice versa. NTPD does NOT do this, does not know about it, and 
doesn't care about it.  Remember NTPD deals with UTC only.  Daylight 
time is, in effect a shift in time zone, even though we don't call it that.

> 
> Mark Berryman
> 
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