[virt-tools-list] Best way to backup my VMs

Decker, Schorschi schorschi.decker at bankofamerica.com
Tue Apr 12 16:37:44 UTC 2011

Correct, that is what we often call in virtualization... a crash consistent state.  This is usually what a VM backup is really, is a crash consistent backup.  Snapshots on active VMs vary according to the Virtualization Platform design, and unless you 'qualify' the file system state against memory state, which is transitionally costly, you have a true gap.  Thus, transactional IO is lost, that is in memory, true that for database systems this can be especially painful.  But most of the time, real-time systems not-withstanding, a crash consistent state, today, is sufficient.


-----Original Message-----
From: virt-tools-list-bounces at redhat.com [mailto:virt-tools-list-bounces at redhat.com] On Behalf Of Christian Grassi
Sent: Tuesday, 12 April, 2011 09:20
To: Frédéric Grelot
Cc: virt-tools-list
Subject: Re: [virt-tools-list] Best way to backup my VMs

Actually you create a doubt in my mind...
If you have a VM with lots of memory, if you just suspend the machine, and back it up without saving the ram, in teory, as no fsync succeded you could lose a lot of stuff which is in the filesystem cache and not synced.
Am I wrong ?


On Tue, 2011-04-12 at 17:50 +0200, Frédéric Grelot wrote:
> > Actually, this isn't right.  Think about the case where you lose 
> > power to your computer; you don't get corrupted disks, you get 
> > "crash-consistent" disks (this is what journaling filesystems are 
> > for).  So if you know what you are doing, it is sufficient just to 
> > pause the guest (virsh suspend <guest>), take the backup, and then 
> > resume the guest (virsh resume <guest>).  True, you won't get all of 
> > the data that might have been in-flight in memory, but it should be 
> > a valid state of the disk.
> I tend to agree with that, it already happened to me for a zimbra server, and I lost nothing.
> > 
> > That all being said, installing backup software in the guest is the 
> > best course of action.
> An other method to profit from virtualization-enabled server would be to take the following actions :
> -inside guest : turn of server application -inside guest : unmount 
> data disk -host : pause guest (necessary?) -host : lvm (or qcow/qed) 
> snapshot of disk -host : resume guest -inside guest : remount and 
> restart server application
> Provided there is no side-effect of unmounting with regard to guest cache (to be confirmed), I think this would reduce downtime, and permits backup of specific parts of the server application without stopping it as a whole.
> A (maybe safest) other option would be to gracefully shutdown the server, LVM/qcow/qed snapshot its disk and restart the server immediately : the data duplication itself won't account for the downtime, since it can be done later.
> Frederic.
> > 
> > --
> > Chris Lalancette
> > 
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> > virt-tools-list at redhat.com
> > https://www.redhat.com/mailman/listinfo/virt-tools-list
> > 
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