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Re: [Qemu-devel] [Qemu-block] migrate -b problems

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [Qemu-block] migrate -b problems
Date: Tue, 18 Apr 2017 15:47:25 +0100
User-agent: Mutt/1.8.0 (2017-02-23)

On Wed, Apr 12, 2017 at 11:18:19AM +0200, Kevin Wolf wrote:
> after getting assertion failure reports for block migration in the last
> minute, we just hacked around it by commenting out op blocker assertions
> for the 2.9 release, but now we need to see how to fix things properly.
> Luckily, get_maintainer.pl doesn't report me, but only you. :-)
> The main problem I see with the block migration code (on the
> destination) is that it abuses the BlockBackend that belongs to the
> guest device to make its own writes to the image file. If the guest
> isn't allowed to write to the image (which it now isn't during incoming
> migration since it would conflict with the newer style of block
> migration using an NBD server), writing to this BlockBackend doesn't
> work any more.
> So what should really happen is that incoming block migration creates
> its own BlockBackend for writing to the image. Now we don't want to do
> this anew for every incoming block, but ideally we'd just create all
> necessary BlockBackends upfront and then keep using them throughout the
> whole migration. Is there a way to get some setup/teardown callbacks
> at the start/end of the migration that could initialise and free such
> global data?

It can be done in the beginning of block_load() similar to
block_mig_state.bmds_list, which is created in init_blk_migration() at
save time.

We can also move the if (blk != blk_prev) blk_invalidate_cache() code
out of the load loop.  It should be done once when setting up

> The other problem with block migration is that is uses a BlockBackend
> name to identify which device is migrated. However, there can be images
> that are not attached to any BlockBackend, or if it is, the BlockBackend
> might be anonymous, so this doesn't work. I suppose changing the field
> to "device name if available, node-name otherwise" would solve this.

Yes, that sounds good and is backwards compatible.

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