WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-users

Re: [Xen-users] Block level domU backup

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


_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users