[libvirt] [RFC] Add a new feild to qemud_save_header to tell if the saved image is corrupt

[Eric Blake]

On 08/17/2011 07:10 AM, Osier Yang wrote:
> If one tries to restore a domain from a corrupt save image, we blindly
> goes forward to restore from it, this can cause many different errors,
> depending on how much the image is saved. E.g.
>
> https://bugzilla.redhat.com/show_bug.cgi?id=730750
>
> So I'm thinking if we can introduce a new feild to struct qemud_save_header,
> such as "bool complete;", and set it true if succeeded to save the image,
> false if not. So that could do some checking while trying to open the image
> (qemuDomainSaveImageOpen), and quit early if "complete" is false, with
> a sensiable error message.

Almost.  I think we can reuse one of the existing spare fields in the 
header (that is, change unused[15] to instead be unused[14] and make the 
new field a uint32_t), and I also think we need to have a tri-state value:

0 - save image was created with older libvirt, no idea if image is sane
1 - save image created by current libvirt, but not yet marked complete; 
attempts to restore from this image should fail with sensible message 
suggesting nuking the save image since it is broken - value written at 
start of save process
2 - save image created by current libvirt and completed - value written 
at end of save process

And of course, we have to update the bswap_header routine to treat it 
with the same endianness as the rest of the header.

>
> Thought?

Sounds reasonable.  I don't even think we have to bump the 
qemud_save_header version (that is, older libvirt will ignore the field, 
and the size is not changing, and newer libvirt will correctly handle 
the situation where the field was not set).

The toughest part will be figuring out how to modify the field when 
using an O_DIRECT fd.  I guess here, we write the entire file O_DIRECT, 
then when qemu completes the save, we close the fd, and open a new one 
that is not O_DIRECT, since we will be polluting the cache with only one 
sector of the file, rather than having to worry about O_DIRECT pickiness 
(the main point of O_DIRECT was for the benefit to the much larger 
portion of the file written by qemu).

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