Re: [Xen-users] Re: [Xen-devel] Re: Writing a tool for Shared Persistent Windows Boot Image

["Andrew Warfield" <andrew.warfield@xxxxxxxxxxxx>]

The problem with this approach is that you end up using two instances
of whatever virtual disk code you want.  In the case of raw writes to
an image file (tap:aio) this is more or less okay, except for the fact
that qemu has a bad habit of buffering writes and so you can get stuck
in a nasty late write race when you switch from emulated writes over
to using pv drivers.

For non-raw writes this is worse, because most of the image file
drivers cache metadata in memory and don't necessarily pick up changes
to the image files when you switch from emulated to pv.  Also, the
disparate implementations make me a bit nervous about headers exactly
matching and so on.

The intention of the phantom device is to try to keep things down to a
single data path, and a single driver implementation.

a.

On 6/29/07, Daniel P. Berrange <berrange@xxxxxxxxxx> wrote:
> On Fri, Jun 29, 2007 at 09:18:50AM -0700, Andrew Warfield wrote:
> > If you can send some more details on the crash we should be able to
> > sort this out -- it's certainly something that has worked in the past.
>
> Ok, so here's the scenario. Traditionally with HVM I have a disk
>
>    file:/xen/rhel4i386.img,hda,w
>
> Having seen the changeset 13827:6524e02edbeb I tried
>
>   tap:aio:/xen/rhel4i386.img,hda,w
>
> And
>
>   tap:aio:/xen/rhel4i386.img,xvda,w
>
> The latter is the preferred, since paravirt drivers should not be hijacking
> the IDE devices inside the guest. However, the changeset 13827 doesn't seem
> to support xvd* since QEMU filters out any devices with such a name.
>
> With vanilla  Xen 3.1.0 qemu goes defunct when starting the guest
> logging
>
>  qemu: could not open hard disk image 'aio:/xen/rhel4i386.img'
>
> After a little investigation I found that in BlktapController
>
>        try:
>            imagetype = self.vm.info['image']['type']
>        except:
>            imagetype = ""
>
> Has long ago been broken and should instead be
>
>         try:
>             imagetype = self.vm.info.image_type()
>         except:
>             imagetype = ""
>
>
> Once I made that change I can see a phantom device being created, but QEMU
> still crashes & burns with  qemu-dm-XXXX.log showing
>
>   qemu: could not open hard disk image '/dev/xvdc1'
>
> I started to debug this, but looking at changeset 13827:6524e02edbeb I rapidly
> came to the conclusion that the whole idea of phantom devices is complete
> overkill & the entire problem could be addressed with a couple of lines in
> ioemu/xenstore.c.  QEMU already knows how to handle all the different
> types of file format blktap does, so there's no need to setup extra phantom
> devices. All thats needed is for QEMU to a) strip the aio: (or equivalent)
> prefix and b) convert xvdN -> hdN if required.
>
> So I'm attaching two patches. The first reverts 13827:6524e02edbeb
> and the second tweaks ioemu/xenstore.c so it can handle blktap devices
> directly. With these applied ontop of Xen 3.1.0 I can successfully
> start HVM guests using  the two example tap:aio lines I show earlier
> in this mail. The patch also adds a couple of logging lines which
> end up in qemu-dm-XXX.log as they'll be useful if ever debugging QEMU
> boot issues - it is far too silent when things go wrong which makes
> diagnosis hard.
>
>     Signed-off-by: Daniel Berrange <berrange@xxxxxxxxxx>
>
>
> The patch being reverted:
>
>  $ diffstat xen-revert-phantom.patch
>  ioemu/xenstore.c                           |   46 ---------------------
>  python/xen/xend/XendConfig.py              |   41 -------------------
>  python/xen/xend/XendDomainInfo.py          |   58 ---------------------------
>  python/xen/xend/server/BlktapController.py |   62 -----------------------------
>  python/xen/xend/server/DevController.py    |   13 ------
>  5 files changed, 3 insertions(+), 217 deletions(-)
>
> The new patch:
>
>  $ diffstat xen-qemu-blktap.patch
>  xenstore.c |   28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
>
> Regards,
> Dan.
> --
> |=- Red Hat, Engineering, Emerging Technologies, Boston.  +1 978 392 2496 -=|
> |=-           Perl modules: http://search.cpan.org/~danberr/              -=|
> |=-               Projects: http://freshmeat.net/~danielpb/               -=|
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