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



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.

Stefan


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