Brian Utterback wrote:
On 12/2/2014 4:00 AM, Rob wrote:
The whole "have 3 servers to select a majority" thing is absolutely not
required when your servers are accurately synchronized themselves and
your requirements are "only" within-a-second. It is true that when you
have two servers the clients cannot know which one is right, but it is
trivial to keep servers within a millisecond of eachother with GPS and
within 10 milliseconds using only network peering. To that is two
orders of magnitude better than you require.
Be careful with this generalization. While it may be "trivial", it isn't
"automatic". I deal with customers all the time that have configured
exactly two servers on their clients and then are surprised later when
all of the clients become unsynchronized and start free drifting. I
always recommend against it. I still think that it takes four to
guarantee a majority but I don't have proof of that. Someday I will
spend some time to either prove or disprove it, but alas, time is
something I don't generally have extra to spend. But you are better off
with one than two from an operational standpoint.
The ntp html docs on selection state that four are needed to
guarantee a majority and give an example of this case.
David
Brian Utterback
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