Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command

[Stefan Hajnoczi]

On Mon, May 20, 2013 at 09:23:43AM +0200, Paolo Bonzini wrote:
> Il 20/05/2013 08:24, Stefan Hajnoczi ha scritto:
> >> > You only need to fdatasync() before every guest flush, no?
> > No, you need to set the dirty bit before issuing the write on the
> > host.  Otherwise the image data may be modified without setting the
> > appropriate dirty bit.  That would allow data modifications to go
> > undetected!
> 
> But data modifications can go undetected until the guest flush returns,
> can't they?

You are thinking about it from the guest perspective - if a flush has
not completed yet then there is no guarantee that the write has reached
disk.

But from a host perspective the dirty bitmap should be conservative so
that the backup application can always restore a bit-for-bit identical
copy of the disk image.  It would be weird if writes can sneak in
unnoticed.

Stefan