[nSLUG] XEN & Heartbeat

Ian Campbell ian at slu.ms
Tue Jun 2 23:20:09 ADT 2009

On Tue, Jun 02, 2009 at 11:08:55PM -0300, Oliver Baltzer wrote:
> Hatem Nassrat wrote:
> > On Tue, Jun 2, 2009 at 10:35 PM, Oliver Baltzer <oliver at hytek.org> wrote:
> >> If you actually know in advance that one of your physical machines will
> >> go away, you can migrate the running VM from one system to the another
> >> online. I have never done this with Xen, but you might even be able to
> > 
> > Slightly offtopic, but which Virtualization Systems allow you to move
> > VM's while keeping them online?
> AFAIK VMWare ESX and KVM. ESX has a whole bunch of infrastructure for
> doing these kinds of things. KVM lets you take a snapshot of the VM,
> which you can then migrate onto a different system and revive there as
> if nothing happened. If you can also teach your network hardware to send
> all packets to the new host you can even keep TCP connections alive. Of
> course the KVM solution is rather hack-ish and in fact the VM is going
> to be gone for a moment, but if that happens before any TCP connection
> times out, you would not notice anything on the client except a small
> delay. When using something like DRBD or a multi-path SAN, such move
> should be relatively quick as you do not need to copy the snapshot to a
> different system, you just need to load it into memory of the new host.
> I have no experience with Xen, but I suspect it is similar.

Xen's is somewhere in between. It requires shared storage (apparently
AoE works, never tried) and does some copy on write magic to sync up
the rest of the environment. I've read in a couple places that there
is a (very) small delay between stopping on one side and starting on
the other, wikipedia says 60-300ms... so assuming you're not right on
the edge of a timeout, your connections should all live.

... so yeah, it kind of sounds like both.

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