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

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Date:	Thu, 16 Jun 2011 14:15:56 -0400
From:	Douglas Gilbert <dgilbert@...erlog.com>
To:	Kay Sievers <kay.sievers@...y.org>
CC:	James Bottomley <James.Bottomley@...senpartnership.com>,
	Greg KH <greg@...ah.com>,
	Nao Nishijima <nao.nishijima.xt@...achi.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	jcm@...hat.com, hare@...e.de, stefanr@...6.in-berlin.de,
	yrl.pp-manager.tt@...achi.com
Subject: Re: [PATCH 1/3] [RFC] genhd: add a new attribute in device structure

On 11-06-16 02:05 PM, Kay Sievers wrote:
> On Thu, Jun 16, 2011 at 20:00, Douglas Gilbert<dgilbert@...erlog.com>  wrote:
>> On 11-06-16 01:20 PM, Kay Sievers wrote:
>>> On Thu, Jun 16, 2011 at 19:09, Kay Sievers<kay.sievers@...y.org>    wrote:
>>>> On Thu, Jun 16, 2011 at 18:25, James Bottomley
>>>> <James.Bottomley@...senpartnership.com>    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?
>>
>> One obvious candidate for a preferred block device name
>> is:
>>   - a SATA disk's WWN (NAA 5 64 bit), or
>>   - a SCSI disk's logical unit name (e.g. SAS: NAA 5)
>>
>> These names (actually numbers) are meant to be world wide
>> unique.
>>
>> The kernel's device naming (following from how devices are
>> discovered) is topological. However at higher levels
>> the user is interested in the device identity. So if
>> unique device names were used as preferred names and
>> preferred names were unique (in a Linux system at any
>> given time) then any subsequent path to an existing device
>> would be highlighted. [That is because subsequent attempts
>> to create its preferred name would fail because it is
>> already there.]
>>
>> You don't need thousands of dollars of equipment to
>> demonstrate this point. An external single disk
>> SATA enclosure with a USB and eSATA interface will do.
>
> Udev does that already since quite a while. This is my cheap laptop:
>    # find /dev/disk/ -name "wwn*"
>    /dev/disk/by-id/wwn-0x50015179593f3038-part1
>    /dev/disk/by-id/wwn-0x50015179593f3038-part4
>    /dev/disk/by-id/wwn-0x50015179593f3038-part3
>    /dev/disk/by-id/wwn-0x50015179593f3038-part2
>    /dev/disk/by-id/wwn-0x50015179593f3038

That is my point, if that disk is eSATA and USB connected
which transport is that link pointing to? I would
prefer eSATA over USB any day but is udev that smart? Or
are we just seeing a symlink to the first (or perhaps last)
path discovered?

Doug Gilbert


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