Re: [Qemu-devel] Killing block migration in qemu?

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/17/2011 10:21 AM, Stefan Hajnoczi wrote:
> On Wed, Aug 17, 2011 at 5:51 PM, Paolo Bonzini<pbonzini@redhat.com>  wrote:
>> following discussions yesterday with Juan Quintela and Marcelo Tosatti, here
>> is my humble proposal: remove block migration from qemu master.  It seems to
>> me that keeping block migration is going to slow down further improvements
>> on migration.  The main problems are:
>>
>> 1) there are very good reasons to move migration to a separate thread. Only
>> a limited amount of extra locking, perhaps none is needed in order to do so
>> for RAM and devices.  But the block drivers pretty much need to run under
>> the I/O thread lock, and coroutines will not help if the I/O thread is taken
>> by another thread.  It's hard/unreliable/pointless to ping-pong migration
>> between threads.
>
> The image streaming approach will also run in the I/O thread for the
> mid-term future.  Is the problem that the block migration code today
> is too tied into the actual migration code path and therefore stops
> from using it when migration happens in a separate thread?

Yes, the problem is that it is much harder to move block migration to a 
separate thread.  It will use a separate coroutine loader and busy loop 
on coroutine locks while holding the iothread.  I'm not even sure it 
won't deadlock.

Block streaming is anyway asynchronous, so it does not hurt that it 
stays in the I/O thread.

>> 2) there already are plans to reimplement block migration... it's called
>> streaming :) and not coincidentially it reuses some of the block migration
>> code.
>
> What are the concrete issues with the existing block migration code?

The code is fine, but it conflicts with a bunch of goals we have for RAM 
live migration.

> This sounds reasonable.  In fact we can do both pre-copy and post-copy
> block migration using streaming (+mirroring).

Right, post-copy is just another name of copy-on-read.  Pre-copy RAM 
migration and post-copy block migration might be the best of both worlds 
in many settings, since pre-copy block migration takes ages.

Paolo