Re: [libvirt] [Qemu-devel] QEMU interfaces for image streaming and post-copy block migration
On 09/07/2010 09:55 AM, Stefan Hajnoczi wrote:
On Tue, Sep 7, 2010 at 3:51 PM, Anthony Liguori
<aliguori(a)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(a)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