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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 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:
   count = stream(device, offset)
   offset += count

Obviously, the "wait_for_idle_time()" requires wide system awareness.
thing I'm not sure about is 1) would libvirt want to expose a similar
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
the question then becomes whether we should try to do this within qemu
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.


Anthony Liguori


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