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

On 09/07/2010 09:34 AM, Kevin Wolf wrote:
Am 07.09.2010 15:41, schrieb Anthony Liguori:

We've got copy-on-read and image streaming working in QED and before
going much further, I wanted to bounce some interfaces off of the
libvirt folks to make sure our final interface makes sense.

Here's the basic idea:

Today, you can create images based on base images that are copy on
write.  With QED, we also support copy on read which forces a copy from
the backing image on read requests and write requests.

In additional to copy on read, we introduce a notion of streaming a
block device which means that we search for an unallocated region of the
leaf image and force a copy-on-read operation.

The combination of copy-on-read and streaming means that you can start a
guest based on slow storage (like over the network) and bring in blocks
on demand while also having a deterministic mechanism to complete the

The interface for copy-on-read is just an option within qemu-img
Shouldn't it be a runtime option? You can use the very same image with
copy-on-read or copy-on-write and it will behave the same (execpt for
performance), so it's not an inherent feature of the image file.

The way it's implemented in QED is that it's a compatible feature. This means that implementations are allowed to ignore it if they want to. It's really a suggestion.

So yes, you could have a run time switch that overrides the feature bit on disk and either forces copy-on-read on or off.

Do we have a way to pass block drivers run time options?

Doing it this way has the additional advantage that you need no image
format support for this, so we could implement copy-on-read for other
formats, too.

To do it efficiently, it really needs to be in the format for the same reason that copy-on-write is part of the format.

You need to understand the cluster boundaries in order to optimize the metadata updates. Sure, you can expose interfaces to the block layer to give all of this info but that's solving the same problem for doing block level copy-on-write.

The other challenge is that for copy-on-read to be efficiently, you really need a format that can distinguish between unallocated sectors and zero sectors and do zero detection during the copy-on-read operation. Otherwise, if you have a 10G virtual disk with a backing file that's 1GB is size, copy-on-read will result in the leaf being 10G instead of ~1GB.

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.
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.
I think libvirt shouldn't have to care about sector offsets. You should
just tell qemu to fetch the image and it should do so. We could have
something like -drive backing_mode=[cow|cor|stream].

This interface let's libvirt decide when the I/O system is idle. The sector is really just a token to keep track of our overall progress.

One thing I envisioned was that a tool like virt-manager could have a progress bar showing the streaming progress. It could update the progress bar based on (offset * 512) / image_size.

If libvirt isn't driving it, we need to detect idle I/O time and we need to provide an interface to query status. Not a huge problem but I'm not sure that a single QEMU instance can properly detect idle I/O time.


Anthony Liguori

A related topic is block migration.  Today we support pre-copy migration
which means we transfer the block device and then do a live migration.
Another approach is to do a live migration, and on the source, run a
block server using image streaming on the destination to move the device.

With QED, to implement this one would:

1) launch qemu-nbd on the source while the guest is running
2) create a qed file on the destination with copy-on-read enabled and a
backing file using nbd: to point to the source qemu-nbd
3) run qemu -incoming on the destination with the qed file
4) execute the migration
5) when migration completes, begin streaming on the destination to
complete the copy
6) when the streaming is complete, shut down the qemu-nbd instance on
the source
Hm, that's an interesting idea. :-)


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