Re: [libvirt] question about save/restore domain

On Tue, Jul 3, 2012 at 9:18 PM, Eric Blake <eblake(a)redhat.com&gt; wrote: > On 07/03/2012 11:35 AM, Jovanka Gulicoska wrote: >> On Tue, Jul 3, 2012 at 6:01 PM, Eric Blake <eblake(a)redhat.com&gt; wrote: >>> >>> On 07/03/2012 06:42 AM, Jovanka Gulicoska wrote: >>>> Hi, >>>> >>>> my name is Jovanka Gulicoska and I'm SoC student. I'm working on >>>> implementing save/load of VM in Gnome-Boxes. >>>> Something confused me about snapshots. In the documentation about life >>>> cycle, in section about Save/Restore is said that domain saved with >>>> virDomainSave/virDomainSaveFlags can be restored only once. >>> >>> Can you point out the URL to that documentation?  It's slightly >>> incorrect, and worth patching to make clearer if that would help the >>> next person to read them. >> >> This is in the draft of the documentation: >> http://libvirt.org/guide/html-single/#Application_Development_Guide-Lifec... >> --- "For basic usage this implies that a guest can only be restored >> once from any given saved state image." > > Ah, and I think reading that sentence in context clarifies the situation: > > "Thus when the guest is restored, the underlying guest storage must be > in exactly the same state as it was when the guest was initially saved. > For basic usage this implies that a guest can only be restored once from > any given saved state image. To allow a guest to be restored from the > same saved state multiple times, the application must also have taken a > snapshot of the guest storage at time of saving, and explicitly revert > to this storage snapshot when restoring." > > Either you manually revert the storage back to match the save image > state before restoring, or you use things in a more basic manner (never > touch the storage manually, and therefore restore works only once > because the storage was unchanged between the save/restore cycle but is > now altered after the successful restore). > > But you're still welcome to provide a patch; the source code for that > page can be found at this repo: >  http://libvirt.org/git/?p=libvirt-appdev-guide.git;a=summary > >>>> So is it better just to use virDomainSnapshotCreateXML instead of >>>> virDomainSave/virDomainSaveFlags? >>> >>> It all depends on what your end goal is with doing a save/load of a VM. >>>  Do you need fast saves, or is it okay if the save takes several >>> seconds?  Will you be loading the state exactly once, or do you plan to >>> make it something that the user can revert to multiple times in a row? >> >> The main idea is to allow the user to save the virtual machine and be >> able to load it when he wants and be able to share it with others who >> uses Boxes, so the other person can also load it on his machine (in >> Boxes). > > Letting _another_ person load the saved image sounds like some degree of > cloning.  Are you trying to clone from an offline state (in which case, > it is just as easy to create a new domain from XML pointing to the > cloned disks), or are you trying to clone from an online state (where > the other user gets to load the domain already booted to the point that > the first user had it)? Yeah, this feature is about cloning the VM but using a single file that has everything about the domain: disk, ram and the config.