Date:	Tue, 16 Oct 2007 15:18:16 -0700
From:	sukadev@...ibm.com
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Greg KH <greg@...ah.com>, Tejun Heo <htejun@...il.com>,
	kay.sievers@...y.org, linux-kernel@...r.kernel.org,
	Linux Containers <containers@...ts.osdl.org>,
	stern@...land.harvard.edu, cornelia.huck@...ibm.com
Subject: Re: [PATCHSET 3/4] sysfs: divorce sysfs from kobject and driver model

Eric W. Biederman [ebiederm@...ssion.com] wrote:
| Greg KH <greg@...ah.com> writes:
| 
| > On Fri, Oct 05, 2007 at 06:12:41AM -0600, Eric W. Biederman wrote:
| >> 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
| >
| > Yes, that is the requirement.
| >
| >> I'm not after changing the user visible major/minor assignments.
| >
| > Oh, I misunderstood what you wrote above then.
| 
| My above sentence is slightly misleading.  That should have been.
| I am not after changing the device name to major:minor assignments
| as specified in Documentation/devices.txt.
| 
| So within a single device namespace everything is normal and as it
| always has been.  Weirdness only ensues when you look across device
| namespaces.
| 
| >> 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.
| >
| > Ok.
| >
| >> 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.
| >
| > Ok.
| >
| >> 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.
| 
| >
| > Ah, but if you do that then the "other" device namespace would have
| > /sys/devices/block/sda/dev be 8:16, right? 
| 
| No. The "other" device namespace I would construct on machine B to
| look just like the device namespace that existed on machine A.
| Making /sys/devices/block/sda would still be 8:0.
| 
| So to be very clear on machine B when talking about disk-1 I would have.
| initial device namespace:
|   /sys/devices/block/sdb
|   /sys/devices/block/sdb/dev 8:16
| 
| "other" device namespace:
|   /sys/devices/block/sda
|   /sys/devices/block/sda/dev 8:0
| 
| Similarly on machine B when talking about disk-2 I would have.
| initial device namespace:
|   /sys/devices/block/sda
|   /sys/devices/block/sda/dev 8:0
| 
| "other" device namespace:
|   /sys/devices/block/sdb
|   /sys/devices/block/sdb/dev 8:16
| 
| So between the two devices namespaces on machine B the two disks
| would exchange their user visible identities.

So an application that would migrate from machine A to B has to
use virtual names (like "disk-1" and "disk-2") to access the disk
right ? 

| 
| Eric
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