On Apr 3, 2007, at 4:21 AM, Morten Torstensen wrote:
Les Mikesell wrote:
Unless something has changed recently, ntp doesn't believe in
multiple sources of time. It will pick the one it thinks is best
and ignore the
Oh, ntp does indeed believe in multiple sources. Sure, only one is
preferred and will be used as timesource, but it is constantly
evaluating the other servers for lag/jitter and change primary
source if it feels like it.
You should have an odd number of servers.. 1, 3, 5, ... and so on.
it appears that Morten, Les, and i are all correct. :)
from "Notes on setting up a NTP subnet" (http://www.eecis.udel.edu/
~mills/ntp/html/notes.html):
--- begin paste ---
Each client in the synchronization subnet (which may also be a server
for other, higher stratum clients) chooses exactly one of the
available servers to synchronize to, usually from among the lowest
stratum servers it has access to. This is, however, not always an
optimal configuration, for indeed NTP operates under another premise
as well, that each server's time should be viewed with a certain
amount of distrust. NTP really prefers to have access to several
sources of lower stratum time (at least three) since it can then
apply an agreement algorithm to detect insanity on the part of any
one of these. Normally, when all servers are in agreement, NTP will
choose the best of these, where "best" is defined in terms of lowest
stratum, closest (in terms of network delay) and claimed precision,
along with several other considerations. The implication is that,
while one should aim to provide each client with three or more
sources of lower stratum time, several of these will only be
providing backup service and may be of lesser quality in terms of
network delay and stratum (i.e., a same-stratum peer which receives
time from lower stratum sources the local server doesn't access
directly can also provide good backup service).
--- end paste ---
in a nutshell: while a NTP client is only using one timeserver at any
given time for synchronization, the client prefers to be able to
select among several different time sources and will switch preferred
servers as it sees fit. thus, while it is perfectly possible to make
NTP work with a single source of time, you are likely to see better
performance if you configure several sources.
the reason i asked about network size is that it seems a bit silly to
configure three time servers for a small home network of, say, four
hosts - you may as well just make every host on your network an NTP
peer, and point them all at each other and at some external time
source. of course, this scheme doesn't scale very well.
-steve
--
If this were played upon a stage now, I could condemn it as an
improbable fiction. - Fabian, Twelfth Night, III,v
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