On 2011-09-16, Richard B. Gilbert <[email protected]> wrote: > On 9/15/2011 10:34 PM, Doug Calvert wrote: >> Hello, >> From time to time people mention that the magic number of servers is >> 4,5,7 and nine. However I can not find a reference or explanation for >> the magical properties. Can anyone explain the background/basis in >> general? And specifically how is five or seven preferable to six? > > Okay! It looks as if you read something I posted here. > > 1. A man with two clocks can never be certain what time it is! > 2. Three clocks are the minimum necessary to get reasonably good and > reliable time. > 3. The weakness of three clocks is that the failure of any one clock > leaves you with case 1.
And the failure or 2 leaves you with 1. And the failure of 3 leaves you with nothing. > 4. Four clocks is the minimum robust configuration. And two clocks going wrong in the same way leaves you with case 1. robust in what sense. As you say below, 4 is not immune to failure of 2. > 5. Five clocks allows for two failed clocks. But the loss of three leaves you with case 1 > 6. Seven clocks allow for the failure of three. > Etc, etc. . . . The only answer is to have at least 10000000000001 clocks although that is also not foolproof:-) You decide what safety you want, and get enough backup to get that safety. > _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
