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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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.

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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