linux-kernel - RE: Time keeps on slipping... on Hyper-V

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Date:	Fri, 26 Sep 2014 06:57:31 +0000
From:	Thomas Shao <huishao@...rosoft.com>
To:	Sitsofe Wheeler <sitsofe@...il.com>
CC:	Olaf Hering <olaf@...fle.de>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"KY Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: Time keeps on slipping... on Hyper-V



> -----Original Message-----
> From: linux-kernel-owner@...r.kernel.org
> [mailto:linux-kernel-owner@...r.kernel.org] On Behalf Of Sitsofe Wheeler
> Sent: Thursday, September 25, 2014 9:28 PM
> To: Thomas Shao
> Cc: Olaf Hering; gregkh@...uxfoundation.org; linux-kernel@...r.kernel.org;
> driverdev-devel@...uxdriverproject.org; apw@...onical.com;
> jasowang@...hat.com; KY Srinivasan; Haiyang Zhang
> Subject: Re: Time keeps on slipping... on Hyper-V
> 
> On Thu, Sep 25, 2014 at 09:40:42AM +0000, Thomas Shao wrote:
> > > > On Tue, Sep 23, Thomas Shao wrote:
> > > >
> > > > with the host clock using host time sample. By default it is
> > > > disabled, because we still recommend user to configure NTP for
> > > > time
> > >
> > > -----Original Message-----
> > > From: Sitsofe Wheeler [mailto:sitsofe@...il.com]
> > > Sent: Thursday, September 25, 2014 2:08 PM
> > >
> > > You [Microsoft?] do? Can you link to public sources where is this
> > > stated
> >
> > As far as I know, currently the document about time sync in Hyper-V is
> > only for Windows. Below is a doc from MSDN:
> > http://blogs.msdn.com/b/virtual_pc_guy/archive/2010/11/19/time-synchro
> > nization-in-hyper-v.aspx
> 
> That post is problematic and not quite enough because it doesn't say "we
> recommend you always configure an external time source for a guest" but
> rather gives a more complicated statement saying "because guest time drifts
> we have a time synchronisation service that will sort things out.
> If you happen to be running a domain controller we recommend you configure
> regular synchronisation to an external time source in addition". This leads to
> questions like: if my (Linux) guest's sole purpose is to run a web server am I in
> the domain controller situation (domain controllers have to re-serve time so
> perhaps they are special)?
> It also doesn't cover the "clock drift is so bad ntpd can't correct it"
> issue.
> 
> Have you seen this document on TechNet -
> http://blogs.technet.com/b/enterprise_admin/archive/2010/04/04/linux-on-hy
> per-v-timesync.aspx
> "With the new Integration Services, Linux virtual machines can!
> 
> TimeSync means that you don’t have to hassle with configur[ing]
> network-based time sync for your Linux VMs on Hyper-V…they can get the
> correct time from the host!"
> 
> This document gives simpler advice looks quite compelling at first glance.
> 
> > We are working on the document about Time Sync on Hyper-V to cover
> > both Linux and Windows.
> 
> The sooner you can get such an official document out the better. All sources of
> guest time synchronisation information (including Microsoft blogs and
> non-Microsoft Q&A sites) need updating so they say the same
> (correct!) thing in a clear way.
> 
> For the meantime can you state what you believe the recommend practice is
> for Linux Hyper-V guests with regards to time synchronisation?
> 

We still recommend user to configure NTP in the guest VM. With the new time sync feature in this patch, 
you could have one more option to enable the guest-host sync, if the NTP didn't work in the environment.
For example the guest VM didn't have network connection.

> --
> Sitsofe | http://sucs.org/~sits/
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