[LDM #TMK-479992]: Monotonic clock confusion

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

Hi Mike,

re:
> We plan to install LDM 6.6.5 on a virtual machine running Linux.

I have done this as a proof of concept on my Windows XP laptop using the Fedora 8,9 and Ubuntu
distributions of Linux.  A couple/few months back a Unidata user in Florida reported using
the same sort of setup on a machine that he is using "operationally".

> 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

An accurate clock is needed most when connections break and are reestablished.  While a
connection is active, the accuracy of the clock is of secondary importance.  The reason
for the need for clock accuracy is so a connection will not miss products during a 
(re)connect process.


>-- 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.

> In addition, our VM software has known issues with system time that are addressed using
> other tools that periodically alter the time in the same manner as ntpd.

In my proof-of-concept testing, I used ntpdate.

> Will these tools interfere with LDM?

No.  A number of Unidata sites still use ntpdate run out of cron to set their clocks.
Again, this is not a problem as long as a feed stays connected.  It can be problematic
if the connection breaks and is reestablished often.

> We would be grateful if you could please clarify this matter for us and pass on any
> advice you might have regarding how we should proceed.

The best thing to do is to try the clock setting procedure recommended by your VM
and see if it suits your needs.
 

Cheers,

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