Re: KVM Block Device Driver

[Fam Zheng <famz@redhat.com>]

On Wed, 08/14 07:29, Spensky, Chad - 0559 - MITLL wrote:
> Stefan, Fam,
> 
>   We are trying to keep an active shadow copy while the system is running
> without any need for pausing.  More precisely we want to log every
> individual access to the drive into a database so that the entire stream
> of accesses could be replayed at a later time.
> 
There's no IO request log infrastructure in QEMU for now. What
drive-mirror can do is to repetitively send changed sectors since last
sending point, but it's not in guest request order or operation size, it
works just with a dirty bitmap.

In your methodology you didn't mention the hook you worked in block.c,
but I think it is necessary to hack block.c to log every r/w access to
the drive, I think you synchronous each write to the shadow image,
right?
> 
> 
> On 8/14/13 6:05 AM, "Stefan Hajnoczi" <stefanha@gmail.com> wrote:
> 
> >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