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

[Stefan Bader <stefan.bader@xxxxxxxxxxxxx>]

On 13.07.2011 12:54, Ian Campbell wrote:
> 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.
> 
> Ian.
> 

It is certainly desirable not to take the major numbers from other subsystems.
And it seems I was confused by switching between older and newer driver
versions. So saying "sda1" in the configuration _was_ already converted into
"xvda1" and it was using the major of xvd. The surprising change was in that
case that "sda1" now converts into "xvde1" (and funnily "hda1" seems to become
"xvda", not "xvda1"). Though I think this is required to have a sane mapping
when people mix hdx and sdx in the configuration.

> 
>> 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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> 
> 
> 
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