On 07/18/2013 11:27 AM, Eric Blake wrote:

>>      if (!has_format) {
>> -        format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : bs->drv->format_name;
>> +        format = mode == NEW_IMAGE_MODE_EXISTING ? NULL : "qcow2";
> 
> Is this the right thing to do?  Or should we do:
> 
> if (!has_format) {
>     if (mode == NEW_IMAGE_MODE_EXISTING) {
>         format = NULL;
>     } else {
>         format = bs->drv->format_name ?: "qcow2";
>     }
> }
> 
> That is, I think we should default to doing a backup in the format given
> by the original (what if the original is qed, which also supports
> backing files), and only use qcow2 when there is no guidance whatsoever.
> 
> But in practice, I don't care

Well, I _DO_ care about one thing - make sure that the qapi-schema.json
page accurately documents how this variable is defaulted for callers
that don't care about the implications of omitting a format.

Or we could simplify life by making 'format' mandatory for drive-backup;
it was optional for 'drive-mirror' due to incremental implementation,
but for 'drive-backup', we still have the opportunity to do things right
from the first release.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org