From: Dor Laor <dlaor@redhat.com>
To: "Thomas Løcke" <thomas.granvej6@gmail.com>
Cc: kvm@vger.kernel.org, Glauber Costa <gcosta@redhat.com>,
Jan Kiszka <jan.kiszka@web.de>
Subject: Re: Time and KVM - best practices
Date: Mon, 22 Mar 2010 11:15:55 +0200 [thread overview]
Message-ID: <4BA7354B.3080501@redhat.com> (raw)
In-Reply-To: <1f0fa7ae1003210429k2e958bfj1b6bd98243cefa84@mail.gmail.com>
On 03/21/2010 01:29 PM, Thomas Løcke wrote:
> Hey,
>
> What is considered "best practice" when running a KVM host with a
> mixture of Linux and Windows guests?
>
> Currently I have ntpd running on the host, and I start my guests using
> "-rtc base=localhost,clock=host", with an extra "-tdf" added for
> Windows guests, just to keep their clock from drifting madly during
> load.
>
> But with this setup, all my guests are constantly 1-2 seconds behind
> the host. I can live with that for the Windows guests, as they are not
Is it just during boot time? If you run ntpdate after the boot inside
the guest, does the time is 100% in sync with the host from that moment on?
Glauber once analyzed it and blames hwclock call in rc.sysinit
> running anything that depends heavily on the time being set perfect,
> but for some of the Linux guests it's an issue.
>
> Would I be better of using ntpd and "-rtc base=localhost,clock=vm" for
> all the Linux guests, or is there some other magic way of ensuring
> that the clock is perfectly in sync with the host? Perhaps there are
> some kernel configuration I can do to optimize the host for KVM?
Jan is the expert here, but last I checked clock=vm is not appropriate
since this is virtual time and not host time - if qemu is
stopped/migrated you won't notice it with virtual time withing the guest
but the drift will grow.
>
> I'm currently using QEMU PC emulator version 0.12.50 (qemu-kvm-devel)
> because version 0.12.30 did not work well at all with Windows guests,
> and the kernel in both host and Linux guests is 2.6.33.1
>
> :o)
> /Thomas
> --
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next prev parent reply other threads:[~2010-03-22 9:14 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-03-21 11:29 Time and KVM - best practices Thomas Løcke
2010-03-22 9:15 ` Dor Laor [this message]
2010-03-22 9:55 ` Alexander Graf
2010-03-22 13:38 ` Glauber Costa
2010-03-22 10:36 ` Jan Kiszka
2010-03-22 12:41 ` Thomas Løcke
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