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xen-devel
[Xen-devel] Re: [RFC, PATCH] docs: Block numbering and naming specificat
On Wed, 2010-09-22 at 12:37 +0100, Stefano Stabellini wrote:
> On Tue, 21 Sep 2010, Ian Jackson wrote:
> > This document describes the vbd device numbering and naming. I've
> > posted versions of it before. It should be in docs/misc, so here is a
> > patch to add it.
> >
> > This is currently an RFC because the section near the bottom about the
> > behaviour of Linux guests needs to be checked for accuracy. In
> > particular, it would be good for Stefano or Jeremy to confirm the
> > behaviour of current pvops kernels.
> >
> > Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > Cc: Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx>
> > Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
> > Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
> >
> > diff -r 77a3da957017 docs/misc/block-numbering-naming.txt
> > --- /dev/null Thu Jan 01 00:00:00 1970 +0000
> > +++ b/docs/misc/block-numbering-naming.txt Tue Sep 21 16:20:53 2010 +0100
> > @@ -0,0 +1,124 @@
> > +Xen guest interface
> > +-------------------
> > +
> > +A Xen guest can be provided with block devices. These are always
> > +provided as Xen VBDs; for HVM guests they may also be provided as
> > +emulated IDE or SCSI disks.
> > +
> > +The abstract interface involves specifying, for each block device:
> > +
> > + * Nominal disk type: Xen virtual disk (aka xvd*, the default); SCSI
> > + (sd*); IDE (hd*).
> > +
> > + For HVM guests, each whole-disk hd* and and sd* device is made
> > + available _both_ via emulated IDE resp. SCSI controller, _and_ as a
> > + Xen VBD. The HVM guest is entitled to assume that the IDE or SCSI
> > + disks available via the emulated IDE controller target the same
> > + underlying devices as the corresponding Xen VBD (ie, multipath).
> > +
> > + For PV guests every device is made available to the guest only as a
> > + Xen VBD. For these domains the type is advisory, for use by the
> > + guest's device naming scheme.
> > +
> > + The Xen interface does not specify what name a device should have
> > + in the guest (nor what major/minor device number it should have in
> > + thee guest, if the guest has such a concept).
> > +
>
> It should be made clear that for HVM guests specifying xvd* in the VM
> config file means that the user is requesting a PV only disk without any
> corresponding emulated disks.
> As a consequence using only xvd* disks in an HVM config file is a
> mistake, because grub (or any other bootloader) wouldn't be able to boot
> the OS.
How does one boot in this case then? The current behaviour is that you
get both xvd* and hd* when you ask for only xvd*. I agree that this is
nasty but it is how it works today so we should at least document what
the correct configuration is if we are going to deprecate it.
Is the correct configuration in this case to have both? e.g.:
disk = ['phy:/dev/VG/VM,xvda,w', 'phy:/dev/VG/VM,hda,w']
Ian
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