Re: [Qemu-devel] [RFC PATCH 0/4] Fix subsection ambiguity in the migration format

[Anthony Liguori <anthony@codemonkey.ws>]

On 07/25/2011 04:10 PM, Paolo Bonzini wrote:
> On Thu, Jun 30, 2011 at 17:46, Paolo Bonzini<pbonzini@redhat.com>  wrote:
> I have now tested this series (exactly as sent) both by examining
> manually the differences between the two formats on the same guest
> state, and by a mix of saves/restores (new on new, 0.14 on new
> pc-0.14, new pc-0.14 on 0.14; also the same combinations on RHEL).  It
> always does what is expected.
>
> Michael Tsirkin objected that the format should be passed as a
> parameter in the migrate command.  I kind of agree, however since this
> is a real bug you would need to bump the default for new machine
> types, and this default would still go in the QEMUMachine struct like
> I am doing.  So I consider the two settings to be orthogonal.  Also,
> the alternative requires changes to the whole management stack and if
> the default is not changed it imposes a broken format unless you
> update the management tools.  Clearly much less bang for the buck.
>
> I think this is ready to go into 0.15.  The bug happens when migrating
> to 0.14 a pc-0.14 machine created with QEMU 0.15 and which has a
> floppy.  The media changed subsection is almost always included, and
> this causes problems when migrating to 0.14 which didn't have any
> subsection for the floppy device.  While QEMU support for migration to
> old version admittedly depends on luck, this isn't true of certain
> downstreams :) which would like to have an unambiguous migration
> format.

I really hate the idea of changing the migration format moments before 
the release.

Since subsections are optional, can't we take the offending subsections, 
remove them, bump the section version numbers and make the fields required?

That "fixes" this issue temporarily without changing the format and we 
can change the format for 1.0.

Regards,

Anthony Liguori

>
> Paolo
>
>