[ntp:questions] NTP servers redundancy

Danny Mayer mayer at ntp.org
Sun Jan 31 03:45:22 UTC 2010

Ryan Malayter wrote:
> On Tue, Jan 19, 2010 at 3:57 PM, Rob <nomail at example.com> wrote:
>> Compare it with a RAID-1 disk system. Â When one disk has an unreadable
>> sector, the situation is clear: use the sector from the other disk.
>> When both disks are readable but return different data, you cannot know
>> which one is correct.
>> This normally is solved by not checking for that condition, rather than
>> to use 3 disks and a majority vote (which still could disagree between
>> all 3 disks).
> Disks use error correcting codes (usually some layered Reed-Solomon
> scheme) at the physical layer to detect errors. Disks rarely, if ever,
> return *incorrect* data. They return known-good data or 'Read failed".

Right. With disks the data stored on it should be the same for all
mirrored disks. If they are not you have a hardware or software problem
with the code that reads and writes to the disks.

> There is no similar error correction code available to discover if an
> individual NTP packet is returning "bad time". Which is why more
> redundant servers are needed.

All servers including a refclock will return a different time no matter
how accurate they are or how close they are. The amount that each one
differs and how they fluctuate from the others is the main part of the
NTP code.


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