Re: [PATCH 1/3] [RFC] genhd: add a new attribute in device structure

[Nao Nishijima]

Hi Hannes,

Thank you for looking at it.

(2011/06/17 15:27), Hannes Reinecke wrote:
> On 06/16/2011 07:20 PM, Kay Sievers wrote:
>> On Thu, Jun 16, 2011 at 19:09, Kay Sievers<kay.sievers@xxxxxxxx>  wrote:
>>> On Thu, Jun 16, 2011 at 18:25, James Bottomley
>>> <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>  wrote:
>>>> On Thu, 2011-06-16 at 09:14 -0700, Greg KH wrote:
>>>>> All userspace naming will be taken care of by the usual udev rules, so
>>>>>> for disks, something like /dev/disk/by-preferred/<fred>  which
>>>>>> would be
>>>>>> the usual symbolic link.
>>>>>
>>>>> No, udev can not create such a link after the preferred name is
>>>>> set, as
>>>>> it has no way of knowing that the name was set.
>>>>
>>>> It can if we trigger a uevent.  Note: I'm not advocating this ...
>>>> I'd be
>>>> equally happy having whatever sets the kernel name create the link (or
>>>> tickle udev to create it).  We definitely require device links, though,
>>>> to get this to work.
>>
>> Guess all that would work now, including mount(8) not canonicalizing.
>> What would happen if we mount:
>>    /dev/disk/by-pretty/foo
>> and some tool later thinks the pretty name should better be 'bar', it
>> writes the name to /sys, we get a uevent, the old link disappears, we
>> get a new link, mount has no device node anymore for the mounted
>> device ...
>>
>> So we basically get a one-shot additional pretty name? Guess, the
>> _single_ name changed anytime later just asks for serious problems. We
>> need to set it very early to be really useful, but how, where is it
>> coming from?
>>
> Well, certain storage arrays are able to print out the user-defined name
> for the LUNs:
> 
> # sg_vpd -p 0xc8 /dev/sdc
> Extended device identification (RDAC) VPD Page:
>   Volume Unique Identifier: 60080e50001bf1f0000005004ddb05a4
>     Creation Number: 1280, Timestamp: Tue May 24 03:11:00 2011
>   Volume User Label: mas-1
>   Storage Array Unique Identifier: 60080e50001bf1f0000000004d418973
>   Storage Array User Label: LSI-SAS-DIF
>   Logical Unit Number: 0000000000000000
> 
> where the 'Volume User Label' is the name the administrator has given to
> the LUN on the storage array.
> 
> So for these kind of things it would be useful.
> 
> However, a single pretty name is quite a limitation.
> And I also fail to see why this can't be handled in userspace.
> 

I know that there are several names for one device.
Preferred name does not eliminate those names, but just add another
pretty name. Usually those machine-generated names are too long and
not familiar with users. preferred name will provide a human-readable
name and reduce the operation cost.

Thanks,

-- 
Nao NISHIJIMA
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., YOKOHAMA Research  Laboratory
Emailï nao.nishijima.xt@xxxxxxxxxxx
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