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

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

Hi, James

(2011/04/09 0:14), James Bottomley wrote:
> On Fri, 2011-04-08 at 07:33 -0700, Hannes Reinecke wrote:
>>> 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.
>> 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.
>>
>> 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.
> 
> So, while I agree totally with the above: udev and userspace is the way
> to go, I'm not totally opposed to having a non-invasive mechanism for
> indicating a user's preferred name for a device.  I think there are a
> couple of ways to do this:
> 
>      1. Entirely in userspace: just have udev consult a preferred name
>         file and create say /dev/disk/by-preferred.  Then have all the
>         tools that normally output device information do the same (i.e.
>         since real name to preferred name is 1:1, they could all do a
>         reverse lookup).
>      2. have a writeable sysfs preferred_name field, either in the
>         generic device or just in SCSI.  The preferred name would be
>         used by outbound only (i.e. kernel dev_printk messages and
>         possibly /proc/partitions).  All inbound uses of the device
>         would come via the standard udev mechanisms
>         (i.e. /dev/disk/by-preferred would be the usual symlink).  This
>         means from the kernel point of view, no renaming has happened.
>         We'd just try to print out the preferred name in certain
>         circumstances, which should solve most of the described problem.
> 
> James
> 
> 
> 

I have a question. Why is in-kernel device name necessary? The kernel
can identify a device by major/miner number and udev can create a
device node of a prefer name.

Currently, device names are only used to show to users. Therefore I
think that in-kernel device name is unnecessary if we introduce your
/dev/disk/by-prefferd idea.


thanks,

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