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



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


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