Re: [Xen-users] Live Migration with different hardware

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I'm brand new to Xen and this list. I'm planning on using Xen. Especialy
> I'm interested in the Live Migration Feature. Somewhere I read that the
> hardware must be identical. What does this mean?
>
> Will migration work from a P4 to a Xeon (Core2 Mircoarchitecure) ? Will
> migration work if they got different NIC? What are the limitations?

The devices won't make a difference (nb. you can only migrate a domain that 
doesn't have direct access to physical devices, but that's the normal case so 
it shouldn't be a problem).  So the network card doesn't matter.

The "identical hardware" stipulation arises from problems with the CPU and the 
instruction set it supports.  When Linux starts up, it tests what 
instructions are available and may choose to use certain instructions that 
are only available on that platform.  Userspace libraries may also do this in 
order to provide optimised implementations of functions.

This is fine if you move to a more featureful processor so e.g. P4 -> Core2 
migration will probably work.  Moving back can break in some cases, however, 
since the kernel and libraries start off using instructions which disappear 
when they migrate back.

It's worth being a bit careful about this.  Often, people can start a domain 
on the less featureful system and then move it to a more featureful system 
*and back* without a crash.  This is because the domain started with the 
smaller featureset and assumes that it stays the same - having booted on the 
less featureful CPU, it moves back successfully.  This may not always work 
safely, since userspace may start using the extra instructions in the 
meantime and then crash when migrated back.

The other thing that bites some people is that, although they sometimes get 
away with migrating a domain back to a less featureful machine, if it reboots 
whilst at the newer machine the kernel will detect the new features and start 
using them.  Then it'll *definitely* crash when migrated back :-(

I believe Core2 has some instructions that P4 doesn't, so migration will 
probably only be reliable in the P4->Core2 direction.

Note that in my above discussion I've mainly spoken about more featureful, or 
newer CPUs.  In practice, the same issues can occur when migrating between 
Intel and AMD CPUs of the same general age and capability, since they might 
implement different instructions for certain functions.

Hope this helps you some.  The same issues are true for live migration, 
non-live migration and save/resume.  Shutting down a guest and then booting 
it on a new host of suitable architecture should always work.

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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