Re: [Xen-devel] incorrect disk numbering with qemu

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 16 Jan 2014, Olaf Hering wrote:
> On Thu, Jan 16, Ian Campbell wrote:
> 
> > Not quite. Each xvd[a-d] creates both a PV and an emulated IDE device
> > hd[a-d], which refer to the same underlying volume.
> > 
> > This allows you to boot from hda, do an unplug and then switch to using
> > xvda.
> 
> Not really: In the end hda is connected to the emulated IDE, so today
> its really "sda" in domU because pata_piix will drive it. sda was
> connected to emulated LSI SCSI. But xvda was not connected to any
> emulated controller, its PV only. Thats how it is done with qemu-trad.
> So having hda and xvda in the same config was working, and maybe even
> supported?
> 
> With qemu-upstream this appearently changed. I'm not saying this change
> in behaviour is good or bad, just that something changed. Some people
> still use the kernel names instead of UUID or LABEL. So they have to
> adjust their config in domU and also in domU.cfg before they switch from
> qemu-trad to qemu-upstream.

This behaviour was chosen on purpose 3 years ago, knowing that it might
cause problems to people that try to use xvda and hda in the same
config.

Why? Because all the alternatives that we considered were far worse.
You can have fun digging through the archives, you might discover things
like xvdHD that might have even made it into Linux for a short while.

Like Ian said, SCSI devices get a name from xvde onward.

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