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

[Pavel Dovgalyuk]

Kevin,

> From: Pavel Dovgalyuk [mailto:address@hidden
> > From: Kevin Wolf [mailto:address@hidden
> > Am 28.09.2016 um 11:32 hat Pavel Dovgalyuk geschrieben:
> > > > From: Kevin Wolf [mailto:address@hidden
> > > > Am 27.09.2016 um 16:06 hat Pavel Dovgalyuk geschrieben:
> > > > > > From: Kevin Wolf [mailto:address@hidden
> > > > > > Am 26.09.2016 um 11:51 hat Pavel Dovgalyuk geschrieben:
> > > > > > > > From: Kevin Wolf [mailto:address@hidden
> > > > > > > > Am 26.09.2016 um 10:08 hat Pavel Dovgalyuk geschrieben:
> > > > > > Originally, we only called bdrv_goto_snapshot() for all _top level_
> > > > > > BDSes, and this is still what you normally get. However, if you
> > > > > > explicitly create a BDS (e.g. with its own -drive option), it is
> > > > > > considered a top level BDS without actually being top level for the
> > > > > > guest, and therefore the snapshotting function is called for it.
> > > > > >
> > > > > > Of course, this is highly inefficient because the goto_snapshot 
> > > > > > request
> > > > > > is passed by the filter driver and then called another time for the
> > > > > > lower node, effectively loading the snapshot a second time.
> > >
> > > Maybe double-saving/loading does the smallest damage then?
> > > And we should just document how to use blkreplay effectively?
> > >
> > > > > >
> > > > > > On the other hand if you use a single -drive option to create both 
> > > > > > the
> > > > > > qcow2 BDS and the blkreplay filter, we do need to pass down the
> > > > > > goto_snapshot request because it won't be called for the qcow2 layer
> > > > > > otherwise.
> > > > >
> > > > > How this can be specified in command line?
> > > > > I believed that separate -drive option is required.
> > > >
> > > > Something like this:
> > > >
> > > >     -drive driver=blkreplay,image.driver=file,image.filename=test.img
> > > >
> > >
> > > I tried the following command line, but VM does not detect the hard drive
> > > and cannot boot.
> > >
> > > -drive 
> > > driver=blkreplay,if=none,image.driver=file,image.filename=testdisk.qcow,id=img-
> > blkreplay
> > > -device ide-hd,drive=img-blkreplay
> >
> > My command line was assuming a raw image. It looks like you're using a
> > qcow (hopefully qcow2?) image. If so, then you need to include the qcow2
> > driver:
> >
> > -drive driver=blkreplay,if=none,image.driver=qcow2,\
> > image.file.driver=file,image.file.filename=testdisk.qcow,id=img-blkreplay
> 
> This doesn't work for some reason. Replay just hangs at some moment.
> 
> Maybe there exists some internal difference between command line with one or 
> two -drive
> options?

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=testdisk.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.

Pavel Dovgalyuk