Re: [Xen-users] time synchronization problem (using NTP)

[Thomas Harold <tgh@xxxxxxxxxxxx>]

Roger Lucas wrote:
> As I understand it, chrony will constantly adjust the system clock 
> time to keep it in line with the remote NTP servers.  It can track 
> frequency drift etc so your system clock changes are as smooth as 
> possible.
That might not be too bad in a DomU. NTP is considered by most to be the
"gold standard" of keeping clocks in sync, but as long as chrony isn't 
jumping the clock forward/backward constantly, it would be acceptable. 
For my uses, I simply want the DomUs to track Dom0 time within a few 
dozen msec (NTP typically keeps the Dom0s all within 2-3 msec on our 
network).
> Don't forget, however, that chrony takes some time (IIRC about 15 
> mins) to synchronise up to the remote servers and accurately 
> calculate the system time error.  Only when it has an accurate 
> estimate of the error will it start to adjust the local system clock.
That happens with NTP as well.  You can speed up the synchronization and
stabilization time period by using "iburst" at the end of the server
lines in ntp.conf.  NTP, when it starts up, will then send off a volley
of packets to the time servers instead of just one, which gives it
enough data to start correcting the clock sooner.  The "iburst"
parameter seems to cause the server clocks to be fairly correct within a
few minutes.  But since the DomUs get their time from Dom0 when they
startup, it's a bit of a moot point (the DomU clocks are hopefully
already accurate, chrony/ntp just need to keep the DomU clocks from
drifting).

...

We originally started with OpenNTPd on our servers (other servers, not
the Xen boxes), but I've spent a lot of time reading the old archives of
comp.protocols.time.ntp and the lengthy online documentation about time
keeping, so we tackled the learning curve of NTP.  So when it came time
to setup the Xen servers, NTP in Dom0 seemed like a natural fit.

Accurate time can be a bit engrossing, there's always the question in
the back of your mind... "can I get it just a little more accurate?".
While I haven't built a GPS reference clock yet for our network, it's
only going to be a matter of time ($85 + misc parts + soldering skills).

...

Once I accomplish a few other things this week, I'll try to come back 
around and play with chrony or the independent_wallclock setting.
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