Re: [Qemu-devel] [PATCH v5 7/9] block: don't make snapshots for filters

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> > I've investigated this issue.
> > This command line works ok:
> >  -drive
> >  driver=blkreplay,if=none,image.driver=file,image.filename=testdisk.qcow,id=img-blkreplay
> >  -device ide-hd,drive=img-blkreplay
> >
> > And this does not:
> >  -drive
> >
> driver=blkreplay,if=none,image.driver=qcow2,image.file.driver=file,image.file.filename=testdis
> k.qcow
> > ,id=img-blkreplay
> >  -device ide-hd,drive=img-blkreplay
> >
> > QEMU hangs at some moment of replay.
> >
> > I found that some dma requests do not pass through the blkreplay driver
> > due to the following line in block-backend.c:
> >     return bdrv_co_preadv(blk->root, offset, bytes, qiov, flags);
> >
> > This line passes read request directly to qcow driver and blkreplay cannot
> > process it to make deterministic.
> 
> I don't understand, blk->root should be the blkreplay here.

I've got some more logs. I used the disk image which references the backing file.
It seems that some weird things happen with both command lines.

== For the first command line (blkreplay separated from image):
blk_co_preadv(img-blkreplay)
 -> bdrv_co_preadv(qcow2, temp_overlay1)
 -> bdrv_co_preadv(blkreplay, temp_overlay)
 -> bdrv_co_preadv(qcow2, temp_overlay2)
 -> bdrv_co_preadv(qcow2, image_overlay)
 -> bdrv_co_preadv(qcow2, image_backing)
 -> bdrv_co_preadv(file, image_backing)

But sometimes it changes to:
blk_co_preadv(img-blkreplay)
 -> bdrv_co_preadv(qcow2, temp_overlay1)
 -> bdrv_co_preadv(file, temp_overlay1)

== For the second command line (blkreplay combined with image):

In most cases we have the following call stack:
blk_co_preadv(img-blkreplay)
 -> bdrv_co_preadv(qcow2, temp_overlay)
 -> bdrv_co_preadv(blkreplay, image_overlay)
 -> bdrv_co_preadv(qcow2, image_overlay)
 -> bdrv_co_preadv(qcow2, image_backing)
 -> bdrv_co_preadv(file, image_backing)

But sometimes it changes to:
blk_co_preadv(img-blkreplay)
 -> bdrv_co_preadv(qcow2, temp overlay)
 -> bdrv_co_preadv(file, temp overlay)

========

It seems, that temporary overlay is created over blkreplay, which
it intended to work as a simple filter. Is that correct?
	
Pavel Dovgalyuk