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aidan
Asked: 2009-11-14 02:23:41 +0800 CST

ntpdate shows 4second time drift / day

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I started ntpdate running every night as a cronjob (on two servers). And every morning I have an email telling me the outcome. On one server it's correcting by about 0.8 seconds every day, which seems like quite a lot to me.

But on the other server, it's correcting by about -4 seconds every day. This sounds like a massive drift. Does this indicate anything wrong, like a low bios battery? (The server is quite old.)

Thanks.

ntp

4 Answers

  1. Best Answer

    A few seconds per day of time drift is not particularly unusual or unexpected on a physical machine in my experience, especially if the drift is fairly stable (i.e. not 1s one day, 10s then next, -2s the day after, ...). In VMs, the clock drifting by only 4s/day would be quite an achievement without clock management like NTP.

    Rather than jumping the time once per day with ntpdate, it is generally a much better idea to run ntpd and have it keep the clock in sync using its much finer grained mechanism. Common "best practise" is to have one machine (or two) that you keep synced with external clock sources such as *.<local>.pool.ntp.org and then sync all your other machine against that - this reduces load on the public NTP servers (as only one or two of your machines are talking to them) and increases the accuracy of the sync between your machines (as they are syncing with the same, local, source).

    If you Linux in any VMWare VMs and want to sync their time by ntp, make sure you are using a recent Linux kernel (that has the "tickless clock"), that VMWare's time sync is off, and that you have tinker panic 0 at the top of the ntpd config so it doesn't give up if the clock skew changes fairly suddenly due to high load in the VM or on the host in general. I've found this the most reliable combination for keeping VM's clocks in order.

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  2. Maybe the server is under heavy load. Also if the server is a virtual machine then heavier load may drift time.

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  3. As far as I know, the battery wont affect the clock while the system is running. RTCs in BIOSes do usually dift quite a bit, but it does depend on various factors. Jeff has covered this topic before in detail.

    I remember a while ago mention that CPU speed stepping can cause clock dift, but I think this has been since fixed.

    Depending on your system usage (maybe time critical applications) it would be worth having ntpd to keep the internal clock in line.

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  4. ntpd should be used where possible, ntpdate can cause problems for instance divide by zero errors in applications.

    ntpd does not change the time, it speeds it up and slows it down to get to the correct time.

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