linux-kernel - Re: [PATCHSET 3/4] sysfs: divorce sysfs from kobject and driver model

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Message-ID: <m1d4vtn4jq.fsf@ebiederm.dsl.xmission.com>
Date:	Fri, 05 Oct 2007 06:12:41 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Greg KH <greg@...ah.com>
Cc:	Tejun Heo <htejun@...il.com>, cornelia.huck@...ibm.com,
	stern@...land.harvard.edu, kay.sievers@...y.org,
	linux-kernel@...r.kernel.org,
	Linux Containers <containers@...ts.osdl.org>
Subject: Re: [PATCHSET 3/4] sysfs: divorce sysfs from kobject and driver model

Greg KH <greg@...ah.com> writes:
>
>>   Also fun is that the dev file implementation needs to be able to
>>   report different major:minor numbers based on which mount of
>>   sysfs we are dealing with.
>
> Um, no, that's not going to happen.  /dev/sda will _always_ have the
> same major:minor number, as defined by the LSB spec.  You can not break
> that at all.  So while you might not want to show all mounts
> /sys/devices/block/sda/ the ones that you do, will all have the LSB
> defined major:minor number assigned to it.

Hmm.  If that is in the LSB it must come from
Documentation/devices.txt  I'm not after changing the user
visible major/minor assignments.

Let me see if a concrete example will help.  Suppose I have
have a SAN with two disks:  disk-1 and disk-2.  I have
two machines A and B.  On machine A I get the mapping:
sda -> disk-1, sdb ->disk-2.  On machine B I wind up with
a different probe order so I get the mapping: sda -> disk-2
sdb ->disk-1.

To be very clear by sda I mean the block device with major 8 and
minor 0, and by sdb I mean the block device with major 8 and minor
16.

So I decide I want an environment on machine B that looks just
like the environment on machine A, so I can bring transfer over
a running program or whatever.  So I run around looking at UUID
labels and what not and I discover that the machine B knows disk-1 as
sdb and that machine A knows disk-1 as sda.  So I want to say:
/sys/devices/block/sdb show up in this other device namespace as
/sys/devices/block/sda.

In that instance a running program won't notice the difference.

Eric
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