[ntp:questions] Isolated Network Drift Problem
cwebster at ec.rr.com
Tue Nov 25 19:39:07 UTC 2008
On Tue, 2008-11-25 at 18:22 +0000, Unruh wrote:
> cwebster at ec.rr.com (Cal Webster) writes:
> >5. Paste the date command into the terminal window exactly when the
> No, you can have it in the window all the time. Just hit return on the 00
Okay, thanks for clarifying that. It's faster for me to copy the
command, along with the carriage return. Then I just click the middle
mouse button to paste at 00 sec.
> >counter turns to the next minute on greenwichmeantime.com.
> >6. Execute "hwclock --systohc" to set the hardware clock.
> >7. Start NTP daemon
> No, do not start the ntp daemon. It is useless in this situation. After a
> few days, run date again comparing it to the time.
Okay, so I'll just be using the difference in seconds between one
invocation of "date" and the next.
How do I translate these seconds into a usable value for "ntptime -f"?
> >##>> Wait 4 days (Thanksgiving day weekend)
> That will give you the averaged rates to about 3PPM at best.
Okay, so would you suggest a week or more? I would be happy to get to
within one second per day right now. We have no critical systems
requiring microsecond accuracy. I just need to keep pretty good time so
logs match up and "Make" dependencies work between NFS mounted resources
on development hosts.
> >[root at fluid root]# cat /etc/ntp/drift
> >##?? Is this a good source to measure the drift?
> >##>> Correct for the drift:
> >ntptime -f 20.196
> No way you will get that kind of accuracy using this procedure.
> You will be lucky with a few PPM
I wasn't aware that I had illustrated any level of accuracy. As I said,
I'm shooting for about 1 sec per day.
> Alternatively you can run chrony, and it will do all of this for you (Ie
> you enter the current time from you wristwatch or whatever, and a few days
> later do so again and again and it will calculate the freq offset and
> offset for you and correct your clock. Then every weekend or day you can enter
> your wristwatch time again and it will keep refining the corrections.
I'll look at "chrony" but I want whatever NTP implementation we use to
be as maintenance free as possible once it's in operation. I don't want
someone to have to manually update anything every day or week.
> Should this be done with ntpd stopped or running? From the man page
> ntp stopped.
Okay, so I'm just going to calculate the difference in seconds between
the system time and my Internet reference, then somehow ("ntptime -f"
maybe?) tell the kernel to use a different frequency offset.
> >it seems this command communicates with the running kernel. Is there a
> >kernel parameter I can set in /etc/sysctl.conf to make this setting
> >persist? The only thing I can see related to the clock in /proc is
> What setting? Note that chrony will also try to estimate the RTC errors for
> you and recalibrate the clock depending on those errors when you start up
> again. Ie, it sounds to me like chrony is a far better tool for your use
> than ntp is.
That's what I'm asking. Is there a kernel parameter to set frequency
offset? If not, how do I make the calculated offset persist across
reboots? Aside from a kernel parameter entry in /etc/sysctl.conf the
only way I can think of is to add the full comand "ntptime -f (offset in
PPM)" to "/etc/rc.local" or "/etc/rc.sysinit".
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