[ntp:questions] question regarding NTP configuration for clusters, and "cluster time" stability

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Sat Nov 21 04:19:00 UTC 2009

Richard B. Gilbert wrote:
> BlackList wrote:
>> rotordyn at yahoo.com wrote:
>>> Look at it this way: We produce a system (in the form
>>>  of a cluster) that works today, but that can drift away
>>>  from UTC since it currently doesn't accept any external
>>>  time reference.  That drift isn't a huge issue, but over
>>>  the multi-year lifespan of the hardware, can be significant.
>>> If we allow the use of external NTP servers, we now have
>>>  opened up what was a closed system, and must prevent that
>>>  new input source from causing instability. ...
>>> An alternate approach would be to make our cluster
>>>  software resilient to intra-node time variations, but it
>>>  was deemed simpler to use NTP to have the cluster accept
>>>  an external UTC source.
>> <http://lopsa.org/node/1480>

The article I referenced seemed relevant to what they
 are attempting.  <http://lopsa.org/node/1480>
  When the external server(s) go away, the internal
    servers take some number of minutes, but eventually
    elect a leader and settle on a usable hierarchy.
   They may drift in total from the absolute notion of NTP
    time, but they remain synchronized with each other.

> There are solutions that do not require that you get time
>  from the internet! ...
> A GPS timing receiver ...
> A VLF receiver ... WWVB ...

They already mentioned several times,
 they aren't going to go that route:

>> A HW time reference isn't feasible, nor should it
>>  really be necessary given our modest accuracy
>>  requirement.
>> Adding hardware isn't an option.
>>  This product exists in the field.
>> With thousands of deployed systems, adding hardware
>>  simply isn't an option.
>> Besides the issue of thousands of systems in use,
>>  there's the added one that we're always deployed
>>  under a roof, never on top of it. :)
>> Adding hardware is not viable. Besides which, it really
>>  is a fairly modest requirement of time accuracy, which
>>  shouldn't require special hardware.
>> And while a hardware time source would cleanly solve the
>>  issue, it isn't feasible to retrofit that to thousands
>>  of existing installations.
>> Even if it were $500, there is the expense of getting it
>>   installed into thousands of commercial data centers.
>>  Which would be a logistical nightmare, given issues of
>>   leased spaces, being deep inside skyscrapers, etc.
>>  It isn't always as simple as sticking a GPS antenna
>>   to a window.

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