On Tue, Jan 19, 2010 at 3:57 PM, Rob <[email protected]> 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.) -- RPM _______________________________________________ questions mailing list [email protected] http://lists.ntp.org/listinfo/questions
