using NTP to check synchronization of twoservers - NTP

This is a discussion on using NTP to check synchronization of twoservers - NTP ; At my work, we're using Amazon EC2 to host all services. The clock is set on our 'instances' by whatever machine the instance is running on -- as is usually the case with Xen. Were all the instances definitely on ...

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Thread: using NTP to check synchronization of twoservers

  1. using NTP to check synchronization of twoservers

    At my work, we're using Amazon EC2 to host all services. The
    clock is set on our 'instances' by whatever machine the
    instance is running on -- as is usually the case with Xen.
    Were all the instances definitely on the same Xen host, I
    could be comfortable saying their clocks were synched pretty
    closely; but at present, that is not the case and it's not
    clear to me how close the clocks will be on instances across
    an EC2 data center (or across data centers). Is there a way to
    run NTP on two servers so that they can calculate their mutual
    time difference?

    --
    _jsn

  2. Re: using NTP to check synchronization of two servers

    Jason Dusek wrote:
    > At my work, we're using Amazon EC2 to host all services. The
    > clock is set on our 'instances' by whatever machine the
    > instance is running on -- as is usually the case with Xen.


    Virtual machines systems tend to run time at a variable rate! There is,
    somewhere a couple of months back, a link to a VMWare paper on how it
    distorts time.

    > Were all the instances definitely on the same Xen host, I
    > could be comfortable saying their clocks were synched pretty
    > closely; but at present, that is not the case and it's not
    > clear to me how close the clocks will be on instances across
    > an EC2 data center (or across data centers). Is there a way to
    > run NTP on two servers so that they can calculate their mutual
    > time difference?


    "ntpdate -d" might be as good, for that application.

    You will be able to set bounds on the time difference, but you won't
    know how much is true difference, and how much is the result of
    asymmetric propagation delays in the NTP packets. On virtual machines,
    scheduling of the virtual machines may introduce significant skewing of
    propagation times. Also, you may find you get almost zero round trip
    times when they were actually quite large, because time was not passing
    on the sending virtual machine.

    If you want approximately right time on a virtual machine, you need to
    NTP synchronise the hosts and run the equivalent of VMWare tools. You
    may find that only the virtual RTC actually has good time.
    >
    > --
    > _jsn


  3. Re: using NTP to check synchronization of two servers

    On 2008-11-03, David Woolley wrote:

    > Jason Dusek wrote:
    >
    >> At my work, we're using Amazon EC2 to host all services. The clock
    >> is set on our 'instances' by whatever machine the instance is
    >> running on -- as is usually the case with Xen.

    >
    > Virtual machines systems tend to run time at a variable rate! There
    > is, somewhere a couple of months back, a link to a VMWare paper on how
    > it distorts time.


    The paper is linked from http://support.ntp.org/bin/view/Support/VMWareNTP

    --
    Steve Kostecke
    NTP Public Services Project - http://support.ntp.org/

  4. Re: using NTP to check synchronization of twoservers

    Thanks to everyone who wrote back.

    --
    _jsn

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