[Xen-devel] Re: Hot backup vs (local) live migration

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

Claris Castillo wrote:
> Hi, this is the third time I post this question to the list but I have not
> received any response.
>
> It is not clear to me why the live migration technique used by Xen and
> documented at
> http://www.cl.cam.ac.uk/research/srg/netos/papers/2005-migration-nsdi-pre.pdf
> can't be used to perform a hot backup of a virtual machine. Based on the
> paper, just  before
> shutting down and destroying the original VM, there is an exact clone of it
> in the remote/destination host.
>  Could this technique be leveraged to perform a hot backup? If so, why Xen
> does not provide with hot
> backup yet? I have been experimenting with VMWare and the performance is
> pretty bad since
> it uses REDO files.
It depends on what your definition of "hot backup is".  Live migration 
while it's active does not result in a consistent machine on the other 
node.  What I mean by this, is that during the live migration process, 
you could not interrupt the migration and just "run" the VM on the 
target node (since only a portion of the memory may have been transfered).
Now, you could basically keep a tight loop of live migrations that would 
result in a backup on another node that would exist at a sightlier 
earlier time than the source node.  That is, your backup may be the 
actual VM 1.5 seconds ago.  The danger here is that if you have an 
application committing transactions to a database, you may end up 
committing the transaction twice after the backup activates (or 
something equally bad).
So the other option is to have what is know as "lock-step" execution 
where the hot backup and the source virtual machine are kept in sync at 
all times.  You usually pay a performance cost for this but you do get 
some pretty interesting fail over.  I'm pretty sure there have been more 
than one research projects devoted to implementing this in Xen.
Regards,

Anthony Liguori

> By the way I tried doing local live migration and I got  *several * error
> messages. I am not sure if the errors
> have to do with the fact that it is not possible to perform a  local live
> migration or some other technical issues
> I can solve if I look at it.
>
>
> I would appreciate any suggestions/advice in this regard.
> cc

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