Re: [Qemu-devel] Re: [patch 2/3] Add support for live block copy

[Avi Kivity]

On 02/23/2011 04:35 PM, Anthony Liguori wrote:
> On 02/23/2011 07:01 AM, Avi Kivity wrote:
> > On 02/23/2011 01:14 AM, Anthony Liguori wrote:
> > > -drive already ties into the qemuopts infrastructure and we have 
> > > readconfig and writeconfig.  I don't think we're missing any major 
> > > pieces to do this in a more proper fashion.
> >
> > The problem with qemu config files is that it splits the 
> > authoritative source of where images are stored into two.  Is it in 
> > the management tool's database or is it in qemu's config file?
>
> I like to use the phrase "stateful config file".  To me, it's just a 
> database for QEMU to persist data about the VM.  It's the only way for 
> QEMU to make certain transactions atomic in the face of QEMU crashing.
>
> The user visible config file is a totally different concept.  A 
> management tool launches QEMU and tells it where to keep it's state 
> database.  The management application may prepopulate the state 
> database or it may just use an empty file.

In that case the word 'config' is misleading.  To me, it implies that 
the user configures something, and qemu reads it, not something mostly 
internal to qemu.

Qemu does keep state.  Currently only images, but in theory also the 
on-board NVRAM.

> QEMU uses the state database to store information that is created 
> dynamically.  For instance, devices added through device_add.  A 
> device added via -device wouldn't necessary get added to the state 
> database.
>
> Practically speaking, it let's you invoke QEMU with a fixed command 
> line, while still using the monitor to make changes that would 
> otherwise require the command line being updated.

Then the invoker quickly loses track of what the actual state is.  It 
can't just remember which commands it issued (presumably in response to 
the user updating user visible state).  It has to parse the stateful 
config file qemu outputs.  But at which points should it parse it?

I don't think it's reasonable to have three different ways to interact 
with qemu, all needed: the command line, reading and writing the 
stateful config file, and the monitor.  I'd rather push for starting 
qemu with a blank guest and assembling (cold-plugging) all the hardware 
via the monitor before starting the guest.

> > For the problem at hand, one solution is to make qemu stop after the 
> > copy, and then management can issue an additional command to 
> > rearrange the disk and resume the guest.  A drawback here is that if 
> > management dies, the guest is stopped until it restarts.  We also 
> > make management latency guest visible, even if it doesn't die at an 
> > inconvenient place.
> >
> > An alternative approach is to have the copy be performed by a new 
> > layered block format driver:
> >
> > - create a new image, type = live-copy, containing three pieces of 
> > information
> >    - source image
> >    - destination image
> >    - copy state (initially nothing is copied)
> > - tell qemu switch to the new image
> > - qemu starts copying, updates copy state as needed
> > - copy finishes, event is emitted; reads and writes still serviced
> > - management receives event, switches qemu to destination image
> > - management removes live-copy image
> >
> > If management dies while this is happening, it can simply query the 
> > state of the copy.  Similarly, if qemu dies, the copy state is 
> > persistent (could be 0/1 or real range of blocks).
>
> This is a more elegant solution to the problem than the commit problem 
> but it's also a one-off.  I think we have a generic problem here and 
> we ought to try to solve it generically (within reason).

Can you give more examples?

I think I demonstrated that hot-plug can be solved via the existing 
interfaces.

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