[vbox-dev] VB backup

Klaus Espenlaub Klaus.Espenlaub at Sun.COM
Tue Mar 30 02:43:44 PDT 2010


Sergey Malykhin wrote:
> Thanks, Nikolay,
> 
> I meant that I hacked kernel a little to get information about file, 
> data buffer, buffer size and offset in file. And it's ok. I'm only not 
> sure that I'll have working backup if do it while VB is running. I 
> understand that the best solution - suspend VB and copy, but it's less 
> interestin, because I'll have pauses in VB work.

Sorry to sound a little annoyed, but I really don't understand why 
adding a little virtualization makes people think they're looking at 
something they never did before.

Creating a copy of the VDI file (no matter how smart is it done by 
intercepting file writes and what not) is just like creating a 
sector-based copy of a hard disk without knowledge of the running guest 
OS. Without taking special measures (for most OSes I wouldn't know what 
to do actually) there cannot be any guarantee that the resulting copy is 
consistent. Even pausing a VM will not fundamentally change this, 
because the guest OS could have unwritten changes in its filesystem 
cache, i.e. in RAM. It'll frequently work without noticeable corruption, 
but it remains a "keep fingers crossed" solution.

The only safe ways to create backups are those which the guest OS is 
aware of (file-based or guest OS based snapshotting like Nikolay 
suggested), or completely shut down the guest OS. Using mechanisms like 
saving state in VirtualBox would require backing up the VM state file as 
well.

Klaus

> Sergey
> 
> 2010/3/30 Nikolay Igotti <Nikolay.Igotti at sun.com 
> <mailto:Nikolay.Igotti at sun.com>>
> 
>      Hi Sergey,
> 
>     Not sure what you mean by "I'm controlling write function to vdi
>     file from kernel", but in general it's possible to use FS's (such as
>     ZFS) snapshotting capability to take snapshots of live VMs image.
>     OTOH to be 100% safe, I would pause VM, flush OS write buffers with
>     'sync', and then copy underlying .vdi.
>     If you assume image to change insignificantly, maybe it would make
>     sense to take snapshot once image stabilized, and only backup
>     differences.
> 
>     Hope that'll help,
>     Nikolay
> 
> 
> 
>     Sergey Malykhin пишет:
> 
>         Hello,
> 
>         I have VB running and want to make backup for vdi file. But I
>         only need to have changed sectors. I'm controlling write
>         function to vdi file from kernel. Can you please answer is it
>         enough just copy it while VM is running or I need to suspend it?
> 
>         TIA
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> 
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-- 
Dr. Klaus Espenlaub
Sun Microsystems GmbH
Werkstrasse 24
71384 Weinstadt
Germany

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