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

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Message-ID: <BANLkTi=eDA1Oa1cVdVNozYu-K5Z0i1uv2g@mail.gmail.com>
Date:	Thu, 16 Jun 2011 20:31:12 +0200
From:	Kay Sievers <kay.sievers@...y.org>
To:	dgilbert@...erlog.com
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 Thu, Jun 16, 2011 at 20:15, Douglas Gilbert <dgilbert@...erlog.com> wrote:
> On 11-06-16 02:05 PM, Kay Sievers wrote:
>>> 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?

I don't know if any bridge supports both connections at the same time.
Mine doesn't.

If two kernel devices fight for the same link, the last one wins,
unless there are link-priorities specified in udev rules.

Kay
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