On Thu, Jun 18, 2015 at 04:49:09PM +0800, Wen Congyang wrote:
> Usage:
> -drive file=xxx,id=Y, \
> -drive file=xxxx,id=X,backing_reference.drive_id=Y,backing_reference.hidden-disk.*
> 
> It will create such backing chain:
>                {virtio-blk dev 'Y'}                                      {virtio-blk dev 'X'}
>                          |                                                          |
>                          |                                                          |
>                          v                                                          v
> 
>     [base] <- [mid] <- ( Y )  <----------------- (hidden target) <--------------- ( X )
> 
>                          v                              ^
>                          v                              ^
>                          v                              ^
>                          v                              ^
>                          >>>> drive-backup sync=none >>>>
> 
> X's backing file is hidden-disk, and hidden-disk's backing file is Y.
> Disk Y may be opened or reopened in read-write mode, so A block backup
> job is automatically created: source is Y and target is hidden disk.
> Active disk X, hidden disk, and Y are all on the same AioContext.

The command-line option name confused me.  I expected "backing_reference" to result in:

  Y <- X

The backup block job and hidden target makes this option more than just
referencing the backing device.

The name "backing.backup-reference" is clearer to me, but I think Kevin
might have more idea for you on this patch.