Re: making snapshots with raw devices? and some general snapshot thoughts

[Tomasz Chmielewski <mangoo@wpkg.org>]

Javier Guerra schrieb:
> On Wed, Feb 25, 2009 at 8:20 AM, Tomasz Chmielewski <mangoo@wpkg.org> wrote:
>> Is it possible to make snapshots when using raw devices (i.e. disk,
>> partition, LVM volume) as guest's disk image?
>>
>> According to documentation[1] (and some tests I made) it is only possible
>> with qcow2 images. Which makes it very inflexible:
>>
>> - one is forced to use a potentially slower file access
>> - one can't use the benefits of i.e. iSCSI disk access, SAN etc.
> 
> what about using 'good' block devices, and add one small, mostly empty
> qcow2? could it be used to store the snapshot for all?  of course it
> would degrade performance while it's active, but should revert after
> 'commiting' it to the 'real' block device(s)

It doesn't work for me - I get:

qm> savevm 1
Error while creating snapshot on 'virtio0'

Where "virtio0" is my "real" block device. I guess it still wants to 
write a snapshot there, as outlined in the documentation:

    "The VM state info is stored in the first qcow2 non removable and
     writable block device. The disk image snapshots are stored in every
     disk image."

Or, am I making a mistake here?

Besides:
- guest will see this second device - not needed
- still, we save the state of disks, but we don't need it for tasks like 
"pausing guest, upgrading kernel on host, rebooting host, resuming guest"


-- 
Tomasz Chmielewski
http://wpkg.org