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(a)linux.vnet.ibm.com&gt; 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