On Mon, Jul 01, 2013 at 05:55:15PM +0200, Peter Lieven wrote: > > Am 01.07.2013 um 16:13 schrieb Stefan Hajnoczi <stefa...@redhat.com>: > > > On Thu, Jun 27, 2013 at 03:11:32PM +0200, Peter Lieven wrote: > > > > This patch breaks cross-version blog migration. We need to control > > whether or not to use the new BLK_MIG_FLAG_ZERO_BLOCK flag. > > you are right the upgrade way works, but downgrade not. what is the > proposed way to fix this?
The safest way is to add a block migration flag that toggles the zero block. By default it would be off. Maybe you would also add a MigrationCapability in qapi-schema.json to indicated that zero blocks are supported in block migration. That would allow management tools to ask QEMU on both sides if they support zero blocks. Use migrate-set-capabilities to enable the zero block feature. CCed Juan Quintela for advice on migration. Stefan