Re: [Qemu-devel] QEMU interfaces for image streaming and post-copy block migration

[Anthony Liguori <aliguori@linux.vnet.ibm.com>]

On 09/07/2010 09:33 AM, Stefan Hajnoczi wrote:
> On Tue, Sep 7, 2010 at 2:41 PM, Anthony Liguori
> <aliguori@linux.vnet.ibm.com>  wrote:
>    
>> The interface for copy-on-read is just an option within qemu-img create.
>>   Streaming, on the other hand, requires a bit more thought.  Today, I have a
>> monitor command that does the following:
>>
>> stream<device>  <sector offset>
>>
>> Which will try to stream the minimal amount of data for a single I/O
>> operation and then return how many sectors were successfully streamed.
>>
>> The idea about how to drive this interface is a loop like:
>>
>> offset = 0;
>> while offset<  image_size:
>>    wait_for_idle_time()
>>    count = stream(device, offset)
>>    offset += count
>>
>> Obviously, the "wait_for_idle_time()" requires wide system awareness.  The
>> thing I'm not sure about is 1) would libvirt want to expose a similar stream
>> interface and let management software determine idle time 2) attempt to
>> detect idle time on it's own and provide a higher level interface.  If (2),
>> the question then becomes whether we should try to do this within qemu and
>> provide libvirt a higher level interface.
>>      
> A self-tuning solution is attractive because it reduces the need for
> other components (management stack) or the user to get involved.  In
> this case self-tuning should be possible.  We need to detect periods
> of I/O inactivity, for example tracking the number of in-flight
> requests and then setting a grace timer when it reaches zero.  When
> the grace timer expires, we start streaming until the guest initiates
> I/O again.
>    

That detects idle I/O within a single QEMU guest, but you might have 
another guest running that's I/O bound which means that from an overall 
system throughput perspective, you really don't want to stream.

I think libvirt might be able to do a better job here by looking at 
overall system I/O usage.  But I'm not sure hence this RFC :-)

Regards,

Anthony Liguori

> Stefan
>