Re: [Xen-users] Block level domU backup

[John Haxby <john.haxby@xxxxxxxxxx>]

Agent Rooker wrote:
> On Tue, Oct 28, 2008 at 1:10 PM, Nick Anderson <nick@xxxxxxxxxxxx> wrote:
>   
> > The one case where I figured there would be a window for corruption
> > was if information was coming over the wire that was being written.
> >     
> So would this be any worse than just yanking the network cable?
>
>
>   
Yes.   When you take a snapshot backup of domU you're basically doing a 
fork() of a running system.   The parent, the original continues and 
runs all its outstanding transactions (receiving mail, sending mail, 
buying CDs from Amazon) and those transactions complete.
When (or if) you restore the snapshot backup those half-completed 
transactions will continue as well.  Anything in-bound will have been 
lost and chances are the worst that will happen is that you'll log a 
message.  Anything outbound will be duplicated -- two copies of a mail 
messages, two CDs from Amazon, credit card debited twice.  Oops.
You can, of course, prepare a system for a snapshot (or fork) so that 
transactions are completed or arrangements made for those important ones 
to not continue in the forked copy.   This isn't a problem unique to 
Xen, of course, you can have this problem with normal, bare-metal 
backups and transactions that are held on disk rather than in memory -- 
it's just that when you do bare-metal backups you're selective about 
what you back up and you don't typically backup things like, for 
example, the sendmail queue.
jch


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