Some of the RADclock papers may be of interest. http://www.synclab.org/docs/ Look for "Virtualize Everything But Time"
On Wed, Oct 9, 2013 at 12:35 AM, Marco Tedaldi <[email protected]>wrote: > > Am 08.10.2013 22:57 schrieb "Ryan Malayter" <[email protected]>: > > > > On Tue, Oct 8, 2013 at 3:41 PM, Arnold Schekkerman > > <[email protected]> wrote: > > > Hi Ryan, > > > What is the advantage of off-host servers? why not use the host as > (single) > > > time-source for all virtual client machines? > > > In a xen environment there is no real need to run the ntpd on "the host". > Since dom0 is basically just another guest, there is in my opinion no need > to use Dom0 as time server. > > Sure, Dom0 must always be available but that's basically all there is... > > > The security policy of most production virutalization environment's > > I've seen explicitly prevents the VMs from talking to the host server > > at all via the network. They're usually on separate VLANs with > > whatever ACLs/firewalls in-between. If you don't have those same sort > > of security requirements, what you describe sounds efficient. > > > And it could be just another DomU (or guests) on xen... > > > A second potential problem is that VMs *move* between hosts while > > they're live and running. So you never really know which physical host > > you're going to be on, so you don't know which server to talk to. > > This could pose some issues. But if you're using an environment like that > than there is for sure some way to provide a bare metal time server. > > I would be interested in how ntp handles beeing moved around like that > anyway (there will for sure be many missed ticks and counters if the VM is > migrated life) > > > Something like VMware DRS moves servers all the time, and even shuts > > down hosts automatically at night to save electricity. So you would > > need some sort of isolated network with the same IP range configured > > in each VM and on each host. Ugly. > > In such an environment you will for sure have a dedicated management LAN > for the VMs and use a jump host for access. Would there be any harm in > using this infrastructure for ntp as well? > > > Or maybe multicast clients with the > > hosts acting as multicast servers. > > > How about broadcast servers? Sure, time keeping is not as good as in > client-server mode but with the low latencies on the virtual LAN... > > > There is always the "time sync" option in the VM tools packages for > > various hypervisors, but that doesn't seem to work as well as running > > NTPd or the Windows Time Service inside the VM in my experience. > > Yes. Time keeping is very hard. Dynamic frequency scaling, multi core > systems and virtualisation do pose some issues. And processor manufacturers > that introduce new counters quite often because they tend to break the > existing ones on a regular basis are not really helping either! > > Best regards, Marco > > _______________________________________________ > pool mailing list > [email protected] > http://lists.ntp.org/listinfo/pool >
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