On 2015-02-23, Brian Inglis <[email protected]> wrote: > On 2015-02-23 10:58, William Unruh wrote: >> On 2015-02-23, Nuno Pereira <[email protected]> wrote: > >>>> On 2015-02-20, Nuno Pereira wrote: >>> By the answers that I've received meanwhile, we need 2 more. > >> You need one more, at a minimum. Ie, with three sources one can alway >> have two outvote the one. four sources are often recommended so that >> even if one dies, or is taken offline you still have three left. Of >> course that reasoning could be extended to say that you need 100000 >> sources, just in case 99997 went offline. >> But with 2, the system is left to hop between them if they diverge. If >> one goes offline of course there is no problem since one always wins in >> a vote with itself the only voter. > > This is why to need to roll patching across your internal time servers > - so that no more than one is offline at one time - and your normal > clients do not lose all their sources at once.
Two sources will do that for you as well. There are two failure modes. One is that a time source goes down. The other is that a time source goes crazy (eg does not impliment leap seconds). Two will protect you against the first, three against the second. Four will protect against both happening at once, etc. For rolling adjustments, three would be fine, as long as you check that all three are behaving properly ( are not false tickers) when you take one of them out briefly. > > Your restricted internal clients' time will start to drift away from UTC > as soon as NTP on their single source goes down for patching. > > If you can get multiple internal sources running with different, independent > external or pool sources, you can peer them in an NTP subnet, and set up a DNS > round robin list name e.g. ntp_pool, to return all internal sources, and > change all > yourinternal clients to use pool instead of server e.g pool ntp_pool iburst > preempt. _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
