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

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Message-ID: <20140925132055.GA13108@sucs.org>
Date:	Thu, 25 Sep 2014 14:28:28 +0100
From:	Sitsofe Wheeler <sitsofe@...il.com>
To:	Thomas Shao <huishao@...rosoft.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

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-synchronization-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-hyper-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?

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