Re: [Xen-users] How does Live Migration work

["Nick Couchman" <Nick.Couchman@xxxxxxxxx>]

>>> On 2009/10/18 at 05:40, Thiago Vinhas <thiago@xxxxxxxxxxxxxxxx>
wrote:
> Hi,
> 
> I've been testing live migration extensively, but I'm not sure how it
works.

It's a pretty complicated process, probably better to ask on the devel
list if you really want details :-).

> 
> Sometimes I can successfully migrate a VM from one server to another,
but 
> other times it just fails, and I don't know the reason.

Any hints in "xm dmesg" or "xm log"?  How about symptoms?  What does it
mean that it "just fails"?  Does it give any error messages?  Does it
give no error messages at all?  Please be more detailed.

> 
> When migrating, xen "saves" the state of the machine, and restores it
on 
> another server, right? Does it saves files somewhere? How much
avaiable 
> memory do I need to run the migration?

I don't think there's actually anything saved to disk, no.  Here's my
understanding of, basically, how it works:
- The domU is created on the destination server.
- An initial copy of the domU's memory is made from source server to
destination server.
- Once the initial copy is done, an incremental copy of memory is made
to see what has changed and the changes are copied over.
- This is done repeatedly until the changes are very, very small.
- The domU is paused on the source side.
- The final incremental memory change is copied over
- The domU is started ("unpaused") on the destination side.

(I could be wrong about something there - I'm not a Xen developer, so I
don't really have a firm grasp on how it works on the code side and
could most definitely be off in my description.)

The last three steps take very little time - milliseconds.  There are
several things that could affect the ability of the domU to migrate
correctly.  Among them are things like network flakiness, speed issues,
etc., when trying to copy the memory over.  Please look at some of the
output from both source and destination systems to determine what is
happening.

-Nick

> 
> Regards,
> -- 
> 
> 
> Thiago Vinhas de Moraes
> ________________________________________________________
> SimpleWeb Tecnologia Inovativa
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