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