Re: Question to recent xentools3 update

[Juergen Hannken-Illjes <hannken%eis.cs.tu-bs.de@localhost>]

On Thu, Mar 27, 2008 at 04:54:13PM +0100, Christoph Egger wrote:
> On Thursday 27 March 2008 15:59:35 Juergen Hannken-Illjes wrote:
> > On Thu, Mar 27, 2008 at 03:51:51PM +0100, Manuel Bouyer wrote:
> > > On Thu, Mar 27, 2008 at 02:18:57PM +0100, Christoph Egger wrote:
> > > > Hi!
> > > >
> > > > In pkgsrc/sysutils/xentools3/patches/patch-di there is a comment,
> > > > which says, reads from raw disk must be sector-aligned.
> > > >
> > > > a) Why is that?
> > >
> > > The raw device pass requests directly to the underlying device, with
> > > only check/adjustements against the partition bounds. Especially it won't
> > > try to do read/modify/write for write requests, or expand the read if
> > > it's not sector-aligned.
> 
> I guess, these aren't desirable in this case.
> 
> > >
> > > > b) Is there a non-raw device w/o this requirement?
> > >
> > > The block device doesn't have this restriction. But it has other side
> > > effects (as it goes though the buffer cache), I don't know if it would be
> > > suitable for pygrub ... it's definitively not usaeable for the block
> > > device backend, or for qemu-dm I/O.
> >
> > As of -current the block device allows only ONE open, therefore it is not
> > usable by pygrub.
> 
> So the problem is, pygrub wants to open the disk multiple times ?
> Or is it that xend has already plumbed in the device to qemu-dm ?
> Is there something that prevents a second open?

Pygrub opens the device once and the kernel doesn't allow a second open.

For my setup with "disk = [ 'phy:/dev/raid0f,0x1,w' ]" I got an EBUSY when
pygrub tried to open the block device.  I don't understand enough of Xen to
say where this disk gets opened (xenbackendd, xbd device ...).

> Isn't it possible to pass pygrub an edited version of the device name or
> have pygrub edit it itself? If we were to use the non-raw device for pygrub
> and the raw device for qemu-dm, would things work?

If a Xen config with "disk = [ 'phy:/dev/rraid0f,0x1,w' ]" works and no other
Xen component opens the corresponding block device this could work.  As my
Xen machine is in production now, I cannot test it.

> Christoph

-- 
Juergen Hannken-Illjes - hannken%eis.cs.tu-bs.de@localhost - TU Braunschweig 
(Germany)