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Re: [Xen-devel] Re: Question on "xen-blkfront: handle Xen major numbers

To: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: Question on "xen-blkfront: handle Xen major numbers other than XENVBD"
From: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
Date: Wed, 13 Jul 2011 11:54:41 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
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On Wed, 2011-07-13 at 11:47 +0100, Stefano Stabellini wrote:
> On Wed, 13 Jul 2011, Stefan Bader wrote:
> > This is /me trying to understand the background of
> > 
> > commit c80a420995e721099906607b07c09a24543b31d9
> > Author: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > Date:   Thu Dec 2 17:55:00 2010 +0000
> > 
> >     xen-blkfront: handle Xen major numbers other than XENVBD
> > 
> > My guess would be that it has its reason from running HVM guests. The issue 
> > I in
> > some way hear in complaints is, that running as PVM guest (at least) people 
> > seem
> > to have used for example "sda1" in the instance configuration and then 
> > relied on
> > the device being called that way within the instance. Now it is suddenly 
> > called
> > "xvde1".
> > This is maybe a broken assumption in the first place, and not that hard to 
> > cope
> > with (its just surprising and maybe breaks some automation). I mainly want 
> > to
> > understand the reasoning, so I can explain it where I get asked.
> The rationale behind this is that it wouldn't be correct for
> xen-blkfront to "steal" the major number of the scsi, sata or ide
> subsystems.

It should be pointed out that the out of mainline "classic" PV Xen
patches do exactly that (although I at least have been discouraging the
use of this configuration for many years now). But it's not something we
felt would fly with upstream.


> Of course if a LABEL or UUID is specified everything should work as
> expected without any changes.
> We do print a warning at boot time to make sure users know of the naming
> change:
>               printk(KERN_INFO "Blkfront and the Xen platform PCI driver have 
> "
>                                       "been compiled for this kernel: unplug 
> emulated disks.\n"
>                                       "You might have to change the root 
> device\n"
>                                       "from /dev/hd[a-d] to /dev/xvd[a-d]\n"
>                                       "in your root= kernel command line 
> option\n");
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