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Posted by Jerry Stuckle on 06/24/07 02:55
Andy Hassall wrote:
> On Sat, 23 Jun 2007 07:28:18 -0400, Jerry Stuckle <jstucklex@attglobal.net>
> wrote:
>
>> Bad programming. What happens, for instance, if the clock on your
>> server is "corrected" back five seconds (possibly because it's running
>> fast)?
>
> Well, that in turn would be bad server administration, since NTP tries to do
> this by slowing the clock to maintain monotonic time - you have to be off by
> minutes before it resorts to going backwards (and even then there's an option
> to prevent it).
>
No, not at all.
First of all, servers do not have clocks with the accuracy of 1 second
in 10M years.
So they have to be adjusted. And even though NTP tries to speed up and
slow the clock - it's not perfect. And at times the clock WILL jump
forwards or backwards at least one second.
Of course, you are assuming:
1) they are running Linux or another Unix variant,
2) NTP server the system is using to set the clock is accurate (not
necessarily the case - most are (at least) second or third tier servers,
3) the network delay between the system and the NTP server is constant
and measurable...
4) The hardware allows the adjustment,
5) The adjustment is within the limits allows (typically 512 ppm, but
this can vary)...
I could continue. The point is - you can't pawn this off on the
administrator. There are a lot of things out of his control.
And programming such that you require the clock to continue incrementing
is a recipe for problems later. No there have not be3en any "unleap
seconds" yet. But that doesn't mean there can't be.
And while I agree with the astronomers that there is no reason for a
negative leap-second, programmers need to be aware it can happen!
--
==================
Remove the "x" from my email address
Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
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