[libvirt] uses of /etc/libvirt/<driver>/

[Daniel P. Berrange]

On Mon, Mar 17, 2014 at 05:46:04PM -0600, Jim Fehlig wrote:
> I received a report about an odd use case of /etc/libvirt/<driver>/
> config files, and would like to hear some opinions about it.  The user
> "preps" a host by mounting a remote fs containing VM images and config,
> creates links in /etc/libvirt/<driver>/dom.xml to
> /mnt-point/whatever/dom.xml, and starts libvirtd.  All is well until
> there is a need to modify the VM config (e.g. virsh setmaxmem ...
> --config), at which point libvirt replaces the link with a file
> containing the new config, instead of updating the contents of the
> linked file.

FWIW the reason why that happens is because when we write the new
XML we don't simply re-write the existing XML file. That would be
unsafe, because in the event of host crash you could end up with
a zero length or partially written XML file. Instead we write to
a temporary file and then issue a rename() syscall which atomically
replaces the old with new.


Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|