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

From: Juan Quintela
Subject: Re: [Qemu-devel] [Qemu-block] migrate -b problems
Date: Wed, 19 Apr 2017 13:18:36 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux)

"Dr. David Alan Gilbert" <address@hidden> wrote:
> * Kevin Wolf (address@hidden) wrote:
>> Am 18.04.2017 um 16:47 hat Stefan Hajnoczi geschrieben:
>> > 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.
>> The difference is that block_load() is the counterpart for
>> block_save_iterate(), not for init_blk_migration(). That is, it is
>> called for each chunk of block migration data, which is interleaved with
>> normal RAM migration chunks.
>> So we can either create each BlockBackend the first time we need it in
>> block_load(), or create BlockBackends for all existing device BBs and
>> BDSes the first time block_load() is called. We still need some place
>> to actually free the BlockBackends again when the migration completes.
>> Dave suggested migration state notifiers, which looked like an option,
>> but at least the existing migration states aren't enough, because the
>> BlockBackends need to go away before blk_resume_after_migration() is
>> called, but MIGRATION_STATUS_COMPLETED is set only afterwards.
>> > 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
>> > BlockBackends.
>> Same problem as above, while saving has setup/cleanup callbacks, we only
>> have the iterate callback for loading.
> Yes, and while we have the notifier chain for the source on migration state
> changes we don't have the notifier on the destination.
> If we just add a load_cleanup  member to SaveVMHandlers and call all of them
> at the end of an inbound migration would that be enough?
> (And define 'end')

We already have a setup() one, that should be enough, no?
We also need a cleanup() one, that is what I am going to add.

Anything else that is needed for this particular problem?

Thanks, Juan.

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