[ntp:questions] NTP servers redundancy

Ryan Malayter malayter at gmail.com
Wed Jan 20 04:57:34 UTC 2010

On Tue, Jan 19, 2010 at 3:57 PM, Rob <nomail at example.com> wrote:
> Compare it with a RAID-1 disk system.  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".

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.

(Okay, a really severe firmware or chip-set bug might return incorrect
data from a disk, but I've not seen much evidence of that in any of
the many disk reliability studies I've read. Sun claims it happens
with cheap disks, which is why ZFS checksums=good. Most of the rest of
the storage industry does not mention the issue.)

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