[libvirt] RFC: add new API to known if domain has been updated

[Daniel P. Berrange]

On Mon, Nov 15, 2010 at 02:02:28PM +0800, Osier Yang wrote:
> 于 2010年11月15日 18:56, Daniel P. Berrange 写道:
> >On Mon, Nov 15, 2010 at 10:26:22AM +0800, Osier Yang wrote:
> >>Hi, all
> >>
> >>Suppose that one updated a domain, and want to restart the
> >>domain with original configuration.
> >>
> >>However, currently, unless shutdown the domain first, and then
> >>restart or reload libvirtd, otherwise domain will still be started
> >>with changed configuration. e.g.
> >>
> >># virsh list --all
> >>  Id Name                 State
> >>----------------------------------
> >>   1 f14                  running
> >>
> >># virsh dumpxml f14 | grep -A 7 cdrom
> >>     <disk type='file' device='cdrom'>
> >>       <driver name='qemu' type='raw'/>
> >>       <source file='/var/lib/libvirt/images/cd1.iso'/>
> >>       <target dev='hdc' bus='ide'/>
> >>       <readonly/>
> >>       <alias name='ide0-1-0'/>
> >>       <address type='drive' controller='0' bus='1' unit='0'/>
> >>     </disk>
> >>
> >># virsh update-device f14 cd2.xml
> >>
> >># virsh dumpxml f14 | grep -A 7 cdrom
> >>     <disk type='file' device='cdrom'>
> >>       <driver name='qemu' type='raw'/>
> >>       <source file='/var/lib/libvirt/images/cd2.iso'/>
> >>       <target dev='hdc' bus='ide'/>
> >>       <readonly/>
> >>       <alias name='ide0-1-0'/>
> >>       <address type='drive' controller='0' bus='1' unit='0'/>
> >>     </disk>
> >>
> >># virsh destroy f14
> >># virsh start f14
> >>
> >># virsh dumpxml f14 | grep -A 7 cdrom
> >>     <disk type='file' device='cdrom'>
> >>       <driver name='qemu' type='raw'/>
> >>       <source file='/var/lib/libvirt/images/cd2.iso'/>
> >>       <target dev='hdc' bus='ide'/>
> >>       <readonly/>
> >>       <alias name='ide0-1-0'/>
> >>       <address type='drive' controller='0' bus='1' unit='0'/>
> >>     </disk>
> >>
> >>it's caused by libvirtd doesn't known if a domain is updated or not,
> >>it updated the domain configuration in memory, but client could
> >>not known about it.
> >>
> >>As a result, one wants to start the domain with original configuration,
> >>he has to shutdown the domain first, and then restart libvirt.
> >
> >This is a bug. libvirt should save a copy of the original configuration
> >at the time the guest starts, so any live changes only remain for the
> >duration of that boot and automatically revert at shutdown.
> 
> yeah, I'm tring to fix it.. :-)
> 
> IMHO it's waste of some memory to let libvirtd save a copy of the
> original domain configuration, though it could just save the
> copy only when updating happens.

Yep, that would be ideal.

> The diffrence with your thought is that it will really relys
> on client, reducing the work load of libvirt, but all the
> clients that using libvirt should be updated with the new APIs
> to fix the bug then, so, not sure if it's fine.

We can fix this entirely in the server side so we can avoid every single
client having to be aware of it.

Even though I don't think this API is the solution to the problem you
describe, I think this API is still useful. There are a couple of 
further enhancements I'd like to see. First add implementation sof
it to all the hypervisor drivers - it should be pretty trivial to
do - the UML/LXC drivers will just be like the QEMU impl. The other
drivers will all just return 0, because they don't have the separate
config in memory vs on disk.

Regards,
Daniel
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