Re: [PATCH 1/2] SCSI: Add a SCSI option for persistent device names

[Nao Nishijima <nao.nishijima.xt@hitachi.com>]

Hi Hannes

(2011/04/08 23:33), Hannes Reinecke wrote:
> On 04/08/2011 07:12 AM, Nao Nishijima wrote:
>> Hi,
>>
>> (2011/04/06 1:14), Greg KH wrote:
>>> On Tue, Apr 05, 2011 at 09:49:46PM +0900, Nao Nishijima wrote:
>>>> This patch series provides a SCSI option for persistent device
>>>> names in kernel. With this option, user can always assign a
>>>> same device name (e.g. sda) to a specific device according to
>>>> udev rules and device id.
>>>>
>>>> Issue:
>>>> Recently, kernel registers block devices in parallel. As a result,
>>>> different device names will be assigned at each boot time. This
>>>> will confuse file-system mounter, thus we usually use persistent
>>>> symbolic links provided by udev.
>>>
>>> Yes, that is what you should use if you care about this.
>>>
>>>> However, dmesg and procfs outputs
>>>> show device names instead of the symbolic link names. This causes
>>>> a serious problem when managing multiple devices (e.g. on a
>>>> large-scale storage), because usually, device errors are output
>>>> with device names on dmesg.
>>>
>>> Then fix your tools that read the output of these files to point to the
>>> proper persistent name instead of trying to get the kernel to solve the
>>> problem.
>>>
>>
>> Yes, the tools should be revised if users get a device name using them.
>>
>> The problem I would like to discuss here is that users can not identify
>> a disk from kernel messages when they DIRECTLY refer to these messages.
>> For example, a device name is used instead of a symbolic link names in
>> bootup messages, I/O devices errors and /proc/partitions …etc.
>>
>> In particular, users can not identify an appropriate device from a
>> device name in syslog since different device name may be assigned to it
>> at each boot time.
>>
>> My idea is able to fix this issue with small changes in scsi subsystem.
>> Also, it is implemented as an option. Therefore, it does not affect
>> users who do not select this option.
>>
> We have been discussing this problem several times in the past, and
> indeed on these very mailing list.
> 
> The conclusion we arrived at is that the kernel-provided device node
> name is inherently unstable and impossible to fix within the existing
> 'sdX' naming scheme.
> So the choices have been to either move to a totally different naming
> scheme or keep the naming scheme and provide the required information
> by other means.
> We have decided on the latter, and agreed on using udev to provide
> persistent device names.

Could you tell me why you chose the latter?


> We are fully aware that any kernel related messages are subject to
> chance after reboot, but then most kernel related messages are
> (PID number, timestamps, login tty etc).
> And also we are aware that any kernel messages need to be matched
> against the current system layout to figure out any hardware-related
> issue.
> 
> But then basically all products requiring to filter out information
> from kernel messages already do so I don't see a problem with that.
> 

All users did not always use the products. Users can see directly
kernel messages (dmesg, /proc/partitions). Therefore I think that
kernel messages need to provide the required mapping.


> Just adding an in-kernel identifier to the LUN will only be an
> incomplete solution, as other identifiers will still be volatile.
> 
> So I would prefer by keeping the in-kernel information as small
> as possible to reduce memory consumption and rely on out-of-band
> programs to provide the required mapping.
> 
> Cheers,
> 

Thanks,

-- 
Nao NISHIJIMA
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., YOKOHAMA Research  Laboratory
Email： nao.nishijima.xt@hitachi.com