Re: [PATCH] qemu: qemu_fopen_fd: differentiate between reader and writer user

[Uri Lublin <uri@il.qumranet.com>]

Avi Kivity wrote:
> Uri Lublin wrote:
>>>>
>>>> Weren't there some acks flowing back on the old protocol to let the 
>>>> source now things are fine?
>>>
>>> There were, but there aren't now.  They don't improve reliability.
>>>
>>
>> Why do you think they don't improve reliability ?
>> The Ack/Go messages catch the scenario where SRC finishes sending all 
>> its state, and DST have a problem loading that state. In that case SRC 
>> thinks migration was completed successfully (and stay in stopped 
>> mode), while DST exists.
> 
> The management tool is in contact with both, and can arbitrate.  Once it 
> thinks both source and destination are in a good state, it can continue 
> the destination and quit the source.
> 

That is true, but in the case I mentioned above it would take the management 
tool some time (guest down time) to realize what happens, and to send "cont" to 
the SRC. With end-of-migration messages SRC discovers DST fails and immediately 
continues.
I agree those messages add some complexity, and slow things a bit for the 
good/average case.