Re: [chrony-users] measuring accuracy of time synchronization among group of servers

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

William G. Unruh __| Canadian Institute for|____ Tel: +1(604)822-3273
Physics&Astronomy _|___ Advanced Research _|____ Fax: +1(604)822-5324
UBC, Vancouver,BC _|_ Program in Cosmology |____ unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 ____|____ and Gravity ______|_ www.theory.physics.ubc.ca/

On Wed, 3 Jun 2020, Simon Kepp wrote:

> I have a cluster of servers KLCEPH01-05 in a lab, that I must maintain
> synchronized clocks on to maintain cluster consistency.clock being
> accurately synced to real time is always a plus, but not a strict
> requirement in this case.
> I've set up Chrony on the servers, and configured all servers with the same
> external upstream source, (currently dk.pool.ntp org, but experimenting to
> find the best source). Furthermore, I've set up all clustermembers to use
> each others as peers. My challenge now is to determine the accuracy to
> which these servers are synchronized relative to each other.
> Output from chronyc sources -v from KLCEPH02 produce the following result:
> root@KLCEPH02:~# chronyc sources -v
> 210 Number of sources = 8
>
>  .-- Source mode  '^' = server, '=' = peer, '#' = local clock.
> / .- Source state '*' = current synced, '+' = combined , '-' = not
> combined,
> | /   '?' = unreachable, 'x' = time may be in error, '~' = time too
> variable.
> ||                                                 .- xxxx [ yyyy ] +/- zzzz
> ||      Reachability register (octal) -.           |  xxxx = adjusted
> offset,
> ||      Log2(Polling interval) --.      |          |  yyyy = measured
> offset,
> ||                                \     |          |  zzzz = estimated
> error.
> ||                                 |    |           \
> MS Name/IP address         Stratum Poll Reach LastRx Last sample
>
> ===============================================================================
> ^+ time.cloudflare.com           3  10   377   489  -1763us[-1763us] +/-
> 12ms
> ^* 5.103.128.88.static.fibi>     1  10   377   510  -1913us[-1891us] +/-
> 7726us
> ^- matrix.rxtx.dk                2  10   377    45  -2797us[-2797us] +/-
> 26ms
> ^+ sweetums.eng.tdc.net          2  10   377   273  -2036us[-2036us] +/-
> 29ms
> =+ klceph01.kepp.tech            2   6   377    21   +459us[ +459us] +/-
> 8107us
> =- KLCEPH03.kepp.tech            4   8   275  478m   +138us[ -647us] +/-
> 9145us
> =- klceph04.kepp.tech            3   6   377    68   -353us[ -353us] +/-
> 8980us
> =+ klceph05.kepp.tech            3   8   373   221   -212us[ -212us] +/-
o
Klceph03 seems to be disconnected. It has been 478 min=30000 sec since it last
responded
so I would exclude that.

also, the readings could be out because your external source is flakey and
your machines are reading it at different times. 
Note that is is miliseconds off and is also pretty unstable. If you really want to test,
get yourself a gps clock and measure how far the clocks are from that gps time
lwhen you connect it to each of the machines. That will give a much better
measure.

You do not say what your requirements for synchronization are. Minutes,
seconds, milliseconds, microseconds, nanoseconds? If you want a few
microseconds using some network source is a bad idea. If you want
milliseconds or longer, then you are probably ok as is.



> 7752us
>
>
> my impression is, that taking abs(xxxx) + abs(zzzz), would be a good
> meassure for the upper bound of how well, the servers are actually in sync
> ( about 8-10 ms in the example above). Is this interpretation correct, or
> is there a better way of determining a bound of their  current internal
> clock accuracy?
>
> Best Regards
> Simon Kepp
> Kepp Technologies

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