How good is NTP for distributed time synchronization?

Question

How accurate is NTP for keeping a set of servers time synchronized?

I'm writing a service which requires a set of servers (some acting as clients, some as servers) synchronized to second level granularity. I'm wondering if NTP is the best thing to use, or if there's something better?

Should I run a ntp server on one of them, and have the others use that as their source? Any other recommendations/horror stories with NTP?

All the servers are linux.

Update: Service levels:

I'd like the one server to be accurate UTC(second level, not microsecond or such), and I'd like all the other servers to be the same ts as that one server, regardless of whether its accurate UTC or not (events are received by this one server from multiple locations at various intervals, I require all those events to be at the same "relative" ts. No, I can't have the main server TS the events as they come in, because that'll require storing an offset (when the event actually happened and when it was logged, which requires a whole lot of extra work), and that complicates matters needlessly.

I've currently set up one server as stratum 2 timeserver, using some startum 1 GPS sources as servers in ntp.conf, on the other servers, I've set this server to be the sole server in ntp.conf.

I hope this will be enough. Thank you!

Answer 1

First off, you might have a look at the Wikipedia NTP page.

Basically, to start with (I preach this regularly) state what the service levels you want might be. Do you need accurate UTC? To what tolerance? That is, do you really need to know what time it is?

Or do you simply want precise synchronization among the systems?

How many machines are we talking about, and are they geographically distributed?

Some options:

• accurate time: Set up at least one server as stratum 2, and have it reference at least 3 stratum 1 servers. If you have lots of servers, make that more than one; obviously you get more reliability by having no single point of failure.

• precise synchronization: set up NTP peers.

• accurate time and geographical distribution: more than one stratum 2 server, as above, with one "near" each cluster; they can peer at stratum 2 to improve the voting.

I don't think there's anything well known better than NTP that's available.

Update Another question mentions the PTP precision time protocol (IEEE 1588) This is excellent for precise synchronization, but depends on multicast.

Also, it's worth considering getting a GPS time source.

Answer 2

NTP will keep you within a second well enough for most applications.

If you need higher precision, and all the servers are running *nix I would investigate implementing Precision Time Protocol. It involves multiple parent clocks and negotiation to find a reliable source in the network. This is the time protocol recommended for timestamping events in the power industry (e.g. accurate timestamping in the log files for relay actions and metering alarms aided in the investigation of the Northeast Blackout of 2003).

Answer 3

Yes, set up one of your servers as your in-house NTP server, and sync the others to that. It gives you accuracy typically within milliseconds, as I remember.

If any of your servers are way off -- and I can't remember what constitutes 'way off' -- NTP won't fix it. There is a way to automatically fix that but I can't remember at the moment.