Xen users - Jul 2007 - Hot backup vs (local) live migration

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.

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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Hi,

the technique that you suggest is in my opinion only suited to protect from
hardware failure.
If within your DomU, something bad happens with your software, you run the
risk that the condition that finally leads to the failure has already
occured when you snapshot your running DomU. Thus, it is quite likely that
the restarted DomU will also fail.

Cheers
dla



On 7/25/07, Claris Castillo <ccastil@us.ibm.com>
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.
>
> 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
>
>
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users
>

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Claris Castillo wrote:
> Hi, this is the third time I post this question to the list but I have not
> received any response.
>   
This is similar to backing up an LVM snapshot of a live system. With 
many operating systems, there may be operations occurring in paging that 
have not yet been committed to the disk. Unless you can shut down things 
like databases and make sure all their content has been committed, you 
should not rely on such a snapshot for backup purposes.
> 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
>
>   
What were your error messages, and how did you try to do the live migration?

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Hi,

First i have a question! Why you need make a memory-ram backup? I read your text
and don`t understand what you are saying exact. The live migration algoritms
"only" manage the data from memory-ram, the data from HD need to be
managed with drdb, iscsi... when we need to backup a xen machine just a
image/lvm/evms needs to be backuped.

Regards,

Grabber.

Em Tue, 24 Jul 2007 21:47:44 -0400
Claris Castillo <ccastil@us.ibm.com> escreveu:
> 
> 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.
> 
> 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
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
_______________________________________________
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http://lists.xensource.com/xen-devel

On Tue, 24 Jul 2007, 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.
I haven''t looked at that PDF, but there''s nothing magic about
a migration.
It copies the memory and CPU state of the guest. In short I copies 
everything you generally would not back up (memory and cpu state), and 
copies nothing that normally you would backup (no files have been copied).

-Tom

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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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