UNIX for Dummies Questions & Answers

It depends on what's in the database, and what kind of database I suppose, but I don't think it's that important.

There are thousands of NTP servers you can try: pool.ntp.org: the internet cluster of ntp servers

You're getting apples and oranges confused. Imagine two identical cars driving on a very long but perfectly staight track. Driver one is simply going 100 MPH and is ignoring driver two. Driver two is trying his best to stay exactly even with driver one. If car one pulls a bit ahead, driver two gently increases speed. If car two pulls a bit ahead, driver two just slows down a bit. With a decent driver in car two, this is not a great trick. Now consider the RPM's on the front tires. The front tires are both cars are going to be spinning very close to the same rate. But how fast is that? Now we need to build a device to display the RPM's of the tires to a human. And if the human is standing beside the road somewhere its even harder to build that RPM display. While the display may not be super accurate, we are sure that the tires are spinning very close to the same rate. Car one is an NTP server. Car two is an NTP client. The tires are the system clocks. That RPM meter is the display clock you don't like.

Perderabo,

I get your example with two car, however, I have to have some trust in a meter, as the meter is all what I see in a car. And it appears at present, that it is all I have to evaluate the time, (rpm) in your example. How can I know that it works as it should, if the meter doesn't show a correct info by definition. Can you see what I mean? How can I say that I trust the NTP server, if my RPM meter is defective?

Breaking away from car thing, how can I tell if the NTP server does a good job? And by the way, there are many NTP servers... So is there one "super NTP" server that feeds its time reading to all other? Should I say "root" NTP server, or may be reference NTP server. Do you prefer some servers over other? Or I'm too concerned about it?

I assume that close we on a net to a server, then better time reading we get, as less delay is introduced by a network.

For better or worse it is very easy to be sure that the tires are spinning at the same rate but much harder to know what the rate is. In the clock example, you need a piece of code to read the clock, convert to a format that you understand, and display it to you. Sadly, all of that takes time. How much depends on how busy the system is. This is where the .2 second come from.

Ideally, you should pick 10 servers or so that seem more or less close to you and tell NTP about all of them. NTP will compare them and separate the false-tickers from the true-chimers (and that is what the NTP docs call them). Network delay is not a big problem. A NTP reply includes the time the request was recieved and the time the reply was sent. The client calculates and compensates for in-transit time.

These days for a few thousand dollars you can get a GPS clock with an ovenized oscillator. You do need to put the antenna on the roof so it gets a good view of the satellites. The satellites know the time to within 30 nanoseconds or so. A decent GPS clock will be accurate to a microsecond or two all the time. We have a few of these and they work pretty well. Just goggle for GPS clocks.

wow, 30 nanoseconds is pretty impressive! Thank for the input, I could never imagine that GPS could be used for precise timekeeping, it is an amazing technology, isn't it? WOW! It practically means, that all Internet timekeeping is done through the space! I could imagine to myself something like an atomic clock, but going with GPS is a wonderful idea!

There's a few of possible time sources out there, not just GPS (which, incidentially, works because it sends the current time). There are receivers for radio transmitted time (like those radio controlled clocks), where the time source at the senders side is (usually) an atomic clock, which in turn present themselves as a Stratum-1 NTP server.

More like GPS requires pretty precise timekeeping to measure the minute differences between signals. Getting a convenient clock beat from them is a handy side-effect.