From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752389AbaIYN2h (ORCPT ); Thu, 25 Sep 2014 09:28:37 -0400 Received: from mail-we0-f174.google.com ([74.125.82.174]:46098 "EHLO mail-we0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751641AbaIYN2g (ORCPT ); Thu, 25 Sep 2014 09:28:36 -0400 Date: Thu, 25 Sep 2014 14:28:28 +0100 From: Sitsofe Wheeler To: Thomas Shao Cc: Olaf Hering , "gregkh@linuxfoundation.org" , "linux-kernel@vger.kernel.org" , "driverdev-devel@linuxdriverproject.org" , "apw@canonical.com" , "jasowang@redhat.com" , KY Srinivasan , Haiyang Zhang Subject: Re: Time keeps on slipping... on Hyper-V Message-ID: <20140925132055.GA13108@sucs.org> References: <1411451081-13423-1-git-send-email-huishao@microsoft.com> <20140923075610.GA10933@aepfle.de> <20140925060745.GA9450@sucs.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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@gmail.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/