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Old 11-13-2008, 01:39 PM
George Magklaras
Default What about Xen? ( NTP problem for virtual RHEL 4 server on VmWare)

We have various paravirtualized (RHEL5 on RHEL5) domains, running MySQL
RDBMSes and a number of other tools for which time synchronization is
critical. What we tend to do is to point (via bridged interfaces) to the
host OS (Dom 0) and then the guests are NTP clients.

However, all the money is on how you start and stop/suspend your guest
OSes. When we start up the array of guests, we make sure that the guest
domains are up, that NTP has started and that time is synced. The way we
do this is by taking various epoch time stamps (see perl localime
function) and look for deviation (simple perl script) by means of yet
another time synced bespoke guest. This can be performed effectively if
you setup a non-privileged ssh key enabled account. As long as the guest
connects to ntpd, it proceeds to perform the check on the rest of the
clients. If that completes successfully, it marshals the start the
mounting of NFS dirs and the start of the RDBMS engines and the rest of
the stuff. In addition, we do not allow users of the guest systems to
shutdown/or suspend them arbitrarily and/or change the scheduling
priority of each domain.

The same check is performed when we are about to shutdown the domains,
all RDBMSes shutdown, NFS systems unmounted and lights off.

This model has worked so far and we have not seen any NTP hickups. To
start an array of say 100 guests, it takes approximately 10-15 minutes
from the point you press click (or the point that all of them start the
domains, all the way to user login and RDBMS access). So, it's not fast,
but it works.


George Magklaras

Senior Computer Systems Engineer/UNIX Systems Administrator
EMBnet Technical Management Board
The Biotechnology Centre of Oslo,
University of Oslo

Miner, Jonathan W (US SSA) wrote:

Changing the subject slightly....

What are peoples' experiences with Xen? I typically run NTP on the physical (host) operating system, and run no time synchronization on the guest operating systems. I don't have any problems... or at least I don't think I have any.

- Jon

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