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

[Anthony Liguori <anthony@codemonkey.ws>]

Tomasz Chmielewski 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.
>
>
> Also, according to the documentation, "VM snapshots are snapshots of 
> the complete virtual machine including CPU state, RAM, device state 
> and the content of all the writable disks".
>
> Which leads to another observation: in some situations what one really 
> needs is to "pause" execution of guest from within the host (i.e. 
> because you want to shutdown or reboot the host due to a kernel or 
> hardware upgrade).
> After all changes on host are done, "guest" should be resumed without 
> even knowing it was paused (most likely it will loose network 
> connections).
>
> This is how it's done with Xen or other virtualization solutions - 
> they only save RAM, CPU, device state to a separate file; they don't 
> save the content of all disks, because it's normally not needed for 
> "pausing" guests.
>
> Is it possible to do it with KVM?

In monitor:

(qemu) stop
(qemu) migrate exec:dd of=state.img

reboot machine

qemu -incoming exec:dd if=state.img -other -options

Regards,

Anthony Liguori
>
>
> [1] http://bellard.org/qemu/qemu-doc.html#SEC18
>
>