making snapshots with raw devices? and some general snapshot thoughts

[Tomasz Chmielewski <mangoo@wpkg.org>]

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?



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


-- 
Tomasz Chmielewski
http://wpkg.org