[LDM #TMK-479992]: Monotonic clock confusion

["Unidata LDM Support" <support-ldm@xxxxxxxxxxxxxxxx>]

Mike,

> > The installation instructions say  that the host machine should have a monotonic clock
> > accurate to within 1 ms. To achieve this, the instructions suggest using ntpd
> >-- which seems like a contradiction because ntpd itself sets the clock back.
> 
> ntpd will adjust the clock.  This can be either setting the clock back or forward.

Just to clarify, when used as intended ntpd(8) adjusts the clock slew rather
than the clock time.  The slew is the first-order derivative with respect to
time of the clock's time: it's the rate at which clock-time passes.  Ideally,
this value should be one; in reality, however, all clocks have values that are
close to, but not exactly, one.  By adjusting the slew, ntpd(8) guarantees
that clock-time will be monotonic.

Having said that, we've seen cases where the slew anomally of a system clock
was so bad (e.g., 10 seconds per hour) that ntpd(8) had no choice but to 
reset the clock -- causing a 0th order discontinuity in time.

I agree with Tom.  Your setup will work.  You might, however, have to
adjust your clock-setting parameters (e.g., how often the clock is reset)
depending on the rate of reconnections.

Regards,
Steve Emmerson

Ticket Details
===================
Ticket ID: TMK-479992
Department: Support LDM
Priority: Normal
Status: Closed