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Re: [Xen-users] Hibernation

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  • Date: Fri, 13 Jul 2007 09:59:51 +0200
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Hello all,

thanks for the clarification. You saved me from reading the authoritative documentation (aka sourcecode) :-)

By default, PV guests share the host's time that cannot be changed from the DomU.

The application layer is no problem as this can be put into the design requirements.
My concern was the one that Mark pointed out and I am glad to hear that PV domains are prepared to handle this.
However, advances in time should be less a problem than stepping backwards, as in that case there might be files that have been created or accessed in the "future".
btw: If you have a look at tools like ntpdate, they can be configured to not correct the time immediately but over multiple iterations that only cause moderate time changes.


On 7/12/07, Dylan Martin <dmartin@xxxxxxxxxxxx> wrote:
Here's another thing.  AFAIK, your running OS probably ignores the
hardware clock.  The only time your system accesses the hw clock is
during boot up to set the system clock and during shutdown to set the
HW clock to match up with the system clock.  Your suspended guest
should need NTP or something to correct the system clock when it wakes

Disclaimer: I'm basing this off of experience setting up NTP servers 5
years ago, with zero exposure to how Windows handles things.  I think
it's the same though.

> > > I am a bit scared that this might cause problems within the affected
> > > OS because after restore, the system clock will show a great leap.
> >
> > IMHO, it would be really weird for an OS, apps or anything to
> > seriously freak out if the clock leaped.  I mean, does your computer
> > crash a daylight savings?  What about when you just set the clock by
> > hand?  If that breaks your OS or apps, you've got bigger problems.
> Well, if apps break they're arguably badly written.  Not that we can probably
> rule that out ;-)  But I wouldn't expect big problems with this; as Dylan
> quite rightly points out there are lots of circumstances when the clock can
> change unexpectedly.
> The case of suspend / resume is a little different, in that if you suspend an
> OS to disk and then resume it later without telling it it /could/ conceivably
> confuse the OS, which might not expect time to suddenly change underneath it.
> For PV, this is fine.  For HVM...  well I assume it works somehow ;-)  I'm
> just not sure how it's currently implemented or if it could maybe be improved
> by making the guest OS more aware of the suspend process.
> Cheers,
> Mark
> --
> Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
> Mark: To answer a question with a question: What use is a skateboard?
> Dave: Skateboards have wheels.
> Mark: My wheel has a wheel!

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