Re: KVM Block Device Driver

[Fam Zheng <famz@redhat.com>]

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.