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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 redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org


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