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

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

On 09/07/2010 09:55 AM, Stefan Hajnoczi wrote:
> On Tue, Sep 7, 2010 at 3:51 PM, Anthony Liguori
> <aliguori@linux.vnet.ibm.com>  wrote:
>    
>> 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 :-)
>>      
> Isn't this what block I/O controller cgroups is meant to solve?  If
> you give vm-1 50% block bandwidth and vm-2 50% block bandwidth then
> vm-1 can do streaming without eating into vm-2's guaranteed bandwidth.
>    

That assumes you're capping I/O.  But sometimes you care about overall 
system throughput more than you care about any individual VM.

Another way to look at it may be, a user waits for a cron job that runs 
at midnight and starts streaming as necessary.  However, the user wants 
to be able to interrupt the streaming should there been a sudden demand.

If the user drives the streaming through an interface like I've 
specified, they're in full control.  It's pretty simple to build a 
interfaces on top of this that implement stream as an aggressive or 
conservative background task too.

>   Also, I'm not sure we should worry about the priority of the I/O too
> much: perhaps the user wants their vm to stream more than they want an
> unimportant local vm that is currently I/O bound to have all resources
> to itself.  So I think it makes sense to defer this and not try for
> system-wide knowledge inside a QEMU process.
>    

Right, so that argues for an incremental interface like I started with :-)

BTW, this whole discussion is also relevant for other background tasks 
like online defragmentation so keep that use-case in mind too.

Regards,

Anthony Liguori

> Stefan
>