Re: KVM Block Device Driver

[Stefan Hajnoczi <stefanha@gmail.com>]

On Wed, Aug 14, 2013 at 10:40:06AM +0800, Fam Zheng wrote:
> On Tue, 08/13 16:13, Spensky, Chad - 0559 - MITLL wrote:
> > Hi All,
> > 
> >   I'm working with some disk introspection on KVM, and we trying to create
> > a shadow image of the disk.  We've hooked the functions in block.c, in
> > particular bdrv_aio_writev.  However we are seeing writes go through,
> > pausing the VM, and the comparing our shadow image with the actual VM
> > image, and they aren't 100% synced up.  The first 1-2 sectors appear to be
> > always be correct, however, after that, there are sometimes some
> > discrepancies.  I believe we have exhausted most obvious bugs (malloc
> > bugs, incorrect size calculations etc.).  Has anyone had any experience
> > with this or have any insights?
> > 
> > Our methodology is as follows:
> >  1. Boot the VM.
> >  2. Pause VM.
> >  3. Copy the disk to our shadow image.
> 
> How do you copy the disk, from guest or host?
> 
> >  4. Perform very few reads/writes.
> 
> Did you flush to disk?
> 
> >  5. Pause VM.
> >  6. Compare shadow copy with active vm disk.
> > 
> >  And this is where we are seeing discrepancies.  Any help is much
> > appreciated!  We are running on Ubuntu 12.04 with a modified Debian build.
> > 
> >  - Chad
> > 
> > -- 
> > Chad S. Spensky
> > 
> 
> I think drive-backup command does just what you want, it creates a image
> and copy-on-write date from guest disk to the target, without pausing
> VM.

Or perhaps drive-mirror.

Maybe Chad can explain what the use case is.  There is probably an
existing command that does this or that could be extended to do this
safely.

Stefan