Re: [libvirt] [RFC] Add a new feild to qemud_save_header to tell if the saved image is corrupt
On 08/17/2011 10:01 AM, Daniel P. Berrange wrote:
The downside to adding a new header field, is that old libvirt
won't look for it. A slightly more evil approach is to
1. Write header, but with 'magic' set to all zerso
Or even to a specific value, different from all zeros, which new libvirt
recognizes as incomplete save file.
2. do migration
3. Re-write header to set correct 'magic'
On the plus side old livirt will refuse to restore from this. On the
downside new libvirt will give less good errors "not a known save
file" instad of "save file is corrupt"
Old libvirt won't recognize the file at all (still a good point, since
it shouldn't be trying to load an incomplete file), but by having two
different magic numbers, newer libvirt can make more sensible error
reporting and decisions on how to handle an incomplete file, in contrast
to a file that is not even a partial save file.
I like the idea of reusing magic rather than burning a new field, since
that buys us protection to older libvirt.
--
Eric Blake eblake(a)redhat.com +1-801-349-2682
Libvirt virtualization library http://libvirt.org